@sheogorath I've just gave it a try, and I could enter-leave fullscreen mode by going through in-game settings.
Anyway, this app doesn't play nice with arbitrary resolutions, so it's probably better avoiding FS and just select an appropriate resolution in the drop-down menu.
@slp interesting, when I tried the in game option I ended up with an reconnection loop of the virt-viewer and the VM display.
Using the menu allowed me to enter full screen mode but I didn't know how to leave. When I figured out Ctrl + alt + f to enter and leave, I noticed that the game crashes by using this shortcut. (Not sure whose problem that is)
But all in all, it's on a good way!