It would be nice if F11 would put focussed VM window into fullscreen, just like many other apps do (Firefox, GNOME Terminal, Evince, Banshee, ...)
This was discussed in the context of virt-viewer recently. See the full details of the decision in bug #508825 Closing as a dup, since the same logic would apply to virt-manager *** This bug has been marked as a duplicate of bug 508825 ***