virtio-gpu redraw issue

virtio-vga device has (re)draw issues in 8.2, - guest display isn't being updated. Depending on the backend (eg sdl/gtk), sometimes it is possible to force it to update by switching to another host window and/or back, or sometimes it just does not (re)draw at all. This is observed on linux host with various guest with -device virtio-vga.

Bisection points to v8.1.0-2857-ga200d53b, - before this commit, guest redraw works fine.

There's another issue open, #2050 (closed), - but this one talks about MacOS guest, which might have its own independent issues, hence I'm filing a new issue for now.