I'm no expert on how it works, but when gir1.2-spice-client-gtk-2.0:amd64 was removed, it was not immediately replaced, and the VM display crashed as soon as I started the VM. The VM was running, as shown in the KVM manager, but no display. When I saw the new spice packages come in yesterday, I gave it a try, and it is working correctly again.
Perhaps there's something about the setup that I was missing, if you were able to keep it running. Are you using spice on a KDE desktop?