Overall changes:
- d/qemu-system-gui-sdl.install: add ui-sdl.so
- d/control: add build dependency
- d/control: add the optional qemu-system-gui-sdl
- d/qemu-system-gui-sdl.install: install ui-sdl.so
GTK is better and generally preferred, but since libsdl2 it isn't too bad and a handful of people each year come up with corner cases were having the ability to run -display sdl would be better.
By adding sdl capability, but holding it back a bit (only a suggest) we could keep the majority of users on GTK but provide others the chance to use SDL when needed.
I have submitted the same for review to a few Ubuntu people and (if not shut down hard) intend to get it into 21.10 there. Just as much as the occasional Ubuntu users that need SDL benefit from this the Debian users would do the same.
I think the combination of libvirt 7.4 (various audio configurations are now selectable and tweakable) + providing GTK (default) and SDL (optional)