Example Packer configuration files for creating a Windows 10 VM on QEMU/KVM/libvirt.

autounattend.xml

<?xml version="1.0" encoding="utf-8"?>
<unattend xmlns="urn:schemas-microsoft-com:unattend">

  <settings pass="windowsPE">

    <component name="Microsoft-Windows-International-Core-WinPE" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
      <SetupUILanguage>
        <UILanguage>en-US</UILanguage>
      </SetupUILanguage>
      <SystemLocale>en-US</SystemLocale>
      <UILanguage>en-US</UILanguage>
      <UserLocale>en-US</UserLocale>
    </component>

    <component name="Microsoft-Windows-Setup" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

      <DiskConfiguration>
        <Disk wcm:action="add">
          <CreatePartitions>
            <CreatePartition wcm:action="add">
              <Order>1</Order>
              <Type>Primary</Type>
              <Extend>true</Extend>
            </CreatePartition>
          </CreatePartitions>
          <ModifyPartitions>
            <ModifyPartition wcm:action="add">
              <Active>true</Active>
              <Extend>false</Extend>
              <Format>NTFS</Format>
              <Letter>C</Letter>
              <Order>1</Order>
              <PartitionID>1</PartitionID>
              <Label>Windows 10</Label>
            </ModifyPartition>
          </ModifyPartitions>
          <DiskID>0</DiskID>
          <WillWipeDisk>true</WillWipeDisk>
        </Disk>
      </DiskConfiguration>

      <ImageInstall>
        <OSImage>
          <InstallTo>
            <DiskID>0</DiskID>
            <PartitionID>1</PartitionID>
          </InstallTo>
        </OSImage>
      </ImageInstall>

      <UserData>
        <AcceptEula>true</AcceptEula>
        <FullName>Packer Admin</FullName>
        <Organization></Organization>
        <ProductKey>
          <Key>W269N-WFGWX-YVC9B-4J6C9-T83GX</Key>
        </ProductKey>
      </UserData>

    </component>

    <component name="Microsoft-Windows-PnpCustomizationsWinPE" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
      <DriverPaths>
        <PathAndCredentials wcm:action="add" wcm:keyValue="1">
          <Path>E:\NetKVM\w10\amd64\</Path>
        </PathAndCredentials>
        <PathAndCredentials wcm:action="add" wcm:keyValue="3">
          <Path>E:\viostor\2k16\amd64\</Path>
        </PathAndCredentials>
      </DriverPaths>
    </component>

  </settings>

  <settings pass="oobeSystem">

    <component name="Microsoft-Windows-Shell-Setup" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">

      <UserAccounts>
        <LocalAccounts>
          <LocalAccount wcm:action="add">
            <Password>
              <Value>packer</Value>
              <PlainText>true</PlainText>
            </Password>
            <Description></Description>
            <DisplayName>Packer Admin</DisplayName>
            <Group>Administrators</Group>
            <Name>packer</Name>
          </LocalAccount>
        </LocalAccounts>
      </UserAccounts>

      <AutoLogon>
        <Password>
          <Value>packer</Value>
          <PlainText>true</PlainText>
        </Password>
        <Enabled>true</Enabled>
        <Username>packer</Username>
      </AutoLogon>

      <OOBE>
        <NetworkLocation>Work</NetworkLocation>
        <HideEULAPage>true</HideEULAPage>
        <ProtectYourPC>3</ProtectYourPC>
        <SkipMachineOOBE>true</SkipMachineOOBE>
        <SkipUserOOBE>true</SkipUserOOBE>
      </OOBE>

      <FirstLogonCommands>
        <SynchronousCommand wcm:action="add">
          <CommandLine>cmd.exe /c powershell -Command "Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Force"</CommandLine>
          <Description>Set Execution Policy 64 Bit</Description>
          <Order>1</Order>
          <RequiresUserInput>true</RequiresUserInput>
        </SynchronousCommand>
        <SynchronousCommand wcm:action="add">
          <CommandLine>cmd.exe /c powershell -File a:\fixnetwork.ps1</CommandLine>
          <Description>Fix public network</Description>
          <Order>2</Order>
          <RequiresUserInput>true</RequiresUserInput>
        </SynchronousCommand>
        <SynchronousCommand wcm:action="add">
          <CommandLine>cmd.exe /c powershell -File "a:\ConfigureRemotingForAnsible.ps1"</CommandLine>
          <Description>Enable WinRM</Description>
          <Order>3</Order>
          <RequiresUserInput>true</RequiresUserInput>
        </SynchronousCommand>
        <SynchronousCommand wcm:action="add">
          <CommandLine>cmd.exe /c reg add "HKLM\System\CurrentControlSet\Control\Network\NewNetworkWindowOff"</CommandLine>
          <Description>Network prompt</Description>
          <Order>4</Order>
          <RequiresUserInput>true</RequiresUserInput>
        </SynchronousCommand>
      </FirstLogonCommands>

    </component>

  </settings>

  <settings pass="specialize">

    <component name="Microsoft-Windows-Shell-Setup" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
      <ComputerName>Win10-Packer</ComputerName>
      <ProductKey>W269N-WFGWX-YVC9B-4J6C9-T83GX</ProductKey>
    </component>

  </settings>

</unattend>

windows10.packer.json

{
    "builders":
    [
        {
            "vm_name": "windows-10",
            "type": "qemu",
            "accelerator": "kvm",

            "cpus": 1,
            "memory": 4096,
            "disk_size": 15360,
      
            "iso_url": "Win10_1909_English_x64.iso",
            "iso_checksum": "86c16116ebacf9b29e4766dd479b5a79",
            "iso_checksum_type": "md5",

            "floppy_files":
            [
                "autounattend.xml",
                "fixnetwork.ps1",
                "ConfigureRemotingForAnsible.ps1"
            ],

            "output_directory": "qemu-drives",
            "qemuargs":
            [
                [ "-drive", "file=qemu-drives/{{ .Name }},if=virtio,cache=writeback,discard=ignore,format=qcow2,index=1" ],
                [ "-drive", "file=./virtio-win.iso,media=cdrom,index=3" ]
            ],

            "communicator": "winrm",
            "winrm_username": "packer",
            "winrm_password": "packer",
            "winrm_use_ssl": "true",
            "winrm_insecure": "true",
            "winrm_timeout" : "1h",

            "shutdown_command": "shutdown /s /t 30 /f",
            "shutdown_timeout": "15m"
        }
    ]
}

If you prefer to use the Git for Windows Bash implementation for the Windows-side shell for SSH you can enable it like so:

        {
            "type": "powershell",
            "inline":
            [
                "$ErrorActionPreference = 'stop'",
                "# Install Chocolatey package manager",
                "Invoke-Expression ((New-Object System.Net.WebClient).DownloadString('https://chocolatey.org/install.ps1'))",
                "# Install git",
                "choco install git -y"
                "# Set bash as OpenSSH shell (as last step, in order not to interfere with powershell provisioners above)",
                "Set-ItemProperty -Path HKLM:\\SOFTWARE\\OpenSSH -Name DefaultShell -Value 'C:\\Program Files\\Git\\bin\\bash.exe' -Force"
            ]
        }