Packer build failed on Xen - QEMU failed to get HVM_PARAM_IOREQ_PFN

packer.log

2018/07/04 12:23:52 ui: ==> qemu: Overriding defaults Qemu arguments with QemuArgs...
2018/07/04 12:23:52 packer: 2018/07/04 12:23:52 Executing /usr/bin/qemu-system-x86_64: []string{"-fda", "/tmp/packer822671350", "-name", "win7x64", "-cdrom", "/home/vagrant/packer_cache/2b74e82a7abe5592aac026c1d0d5da01dd5a8dfad148b31b32203d23739da902.iso", "-smp", "1", "-m", "2048M", "-machine", "type=pc,accel=xen", "-device", "e1000,netdev=user.0", "-vnc", "0.0.0.0:23", "-netdev", "user,id=user.0,hostfwd=tcp::2387-:5985", "-drive", "file=output-qemu/win7x64,if=ide,cache=writeback,discard=ignore,format=qcow2", "-boot", "once=d"}
2018/07/04 12:23:52 packer: 2018/07/04 12:23:52 Started Qemu. Pid: 10999
2018/07/04 12:23:53 packer: 2018/07/04 12:23:53 Qemu stderr: WARNING: Image format was not specified for '/tmp/packer822671350' and probing guessed raw.
2018/07/04 12:23:53 packer: 2018/07/04 12:23:53 Qemu stderr:          Automatically detecting the format is dangerous for raw images, write operations on block 0 will be restricted.
2018/07/04 12:23:53 packer: 2018/07/04 12:23:53 Qemu stderr:          Specify the 'raw' format explicitly to remove the restrictions.
2018/07/04 12:23:53 packer: 2018/07/04 12:23:53 Qemu stderr: failed to get HVM_PARAM_IOREQ_PFN
2018/07/04 12:23:53 packer: 2018/07/04 12:23:53 Qemu stderr: qemu-system-x86_64: failed to get ioreq server info: error 22 handle=0x56342ec0c710
2018/07/04 12:23:53 packer: 2018/07/04 12:23:53 Qemu stderr: qemu-system-x86_64: xen hardware virtual machine initialisation failed
2018/07/04 12:23:53 ui error: ==> qemu: Error launching VM: Qemu failed to start. Please run with PACKER_LOG=1 to get more info.
==> qemu: Error launching VM: Qemu failed to start. Please run with PACKER_LOG=1 to get more info.
==> qemu: Deleting output directory...
2018/07/04 12:23:53 packer: 2018/07/04 12:23:53 Deleting floppy disk: /tmp/packer822671350
2018/07/04 12:23:53 ui: ==> qemu: Deleting output directory...
Build 'qemu' errored: Build was halted.
2018/07/04 12:23:53 [INFO] (telemetry) ending qemu
2018/07/04 12:23:53 ui error: Build 'qemu' errored: Build was halted.
2018/07/04 12:23:53 Builds completed. Waiting on interrupt barrier...
2018/07/04 12:23:53 machine readable: error-count []string{"1"}
2018/07/04 12:23:53 ui error: 
==> Some builds didn't complete successfully and had errors:
2018/07/04 12:23:53 machine readable: qemu,error []string{"Build was halted."}
2018/07/04 12:23:53 ui error: --> qemu: Build was halted.
2018/07/04 12:23:53 ui: 
==> Builds finished but no artifacts were created.
2018/07/04 12:23:53 [INFO] (telemetry) Finalizing.

==> Some builds didn't complete successfully and had errors:
--> qemu: Build was halted.

==> Builds finished but no artifacts were created.
2018/07/04 12:23:53 waiting for all plugin processes to complete...
2018/07/04 12:23:53 /usr/local/bin/packer: plugin process exited
2018/07/04 12:23:53 /usr/local/bin/packer: plugin process exited

This seems to be related to a thread I have going on the Xen-users mailing list. The gist has been that Xen isn't aware it needs to start an HVM environment because qemu isn't telling the hypervisor this.