Debian or Kali Linux installed to as KVM (libvirtd) guests do not automatically have qemu-guest-agent or spice-vdagent installed. This will prevent seamless movement of the mouse cursor between the guest and host desktop in Virtual Machine Manager (requiring the use of a Ctrl-Alt to release the cursor from the guest window).
To cure this, install both qemu-guest-agent and spice-vdagent on each guest and reboot (the guests).
sudo apt install qemu-guest-agent
sudo apt install spice-vdagent
sudo apt install xserver-xorg-video-qxl