[sdl] Mouse grab breaks GNOME 3 screensaver unlock screen

When the GNOME 3 screensaver activates with the mouse cursor over the SDL window, the screensaver will not unlock unless the gnome-shell process is killed and restarted manually. This seems to be related to the fact that the key strokes are grabbed by SDL, but the screensaver will not allow the Ctrl and Alt keys to be passed to QEMU to exit the mouse grab.

Qemu-kvm 1.0.1, QEMU 1.0.1, kernel 3.2.15.