Comment 12 for bug 1906245

For those in a hurry that just want things to work again here a few workarounds for now:

Workaround #1 - uninstall and install
$ apt remove qemu-system-gui
# the remove will complain the same way as the upgrade, but an install
# of the new version over this state "rH" works.
# You most likely had this installed through qemu-system-x86
# so remember to re-install dependency removed packages as well:
$ apt install qemu-system-x86 qemu-system-gui

Workaround #2 - use repackaged 5.1+dfsg-4ubuntu1 (If you can't remove qemu* packages for some reason)
# replace original 5.1+dfsg-4ubuntu1 with one that does not have the prerm issue
$ wget https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/1906245/+attachment/5439620/+files/qemu-system-gui_5.1+dfsg-4ubuntu1_amd64.repackaged.lp1906245.deb
$ dpkg -i qemu-system-gui_5.1+dfsg-4ubuntu1_amd64.repackaged.lp1906245.deb
# upgrade to 5.1+dfsg-4ubuntu2 (now working)
$ apt install qemu-system-gui