Question: are you running the default Debian kernel (in Whonix)? I saw this with newer kernel versions I compiled for guests. If the former is true then its a libvirt bug only.
I reported this upstream but in vain. Because we are a virtualized OS we have the misfortune of running into these bugs before everyone else.
This is another problem recently fixed upstream but unfortunately it will take some time to come into Debian stable.
It was so annoying that we created a package that fixes it. Please follow the instructions to install the qxl-xorg-enhance package:
BTW what libvirt/KVM versions are you running?