gtk: Use ctrl+alt+q for quit accelerator

Using the standard ctrl+q makes it too easy to kill the whole VM. Using
ctrl+alt+FOO is consistent with our other accelerators.

https://bugzilla.redhat.com/show_bug.cgi?id=1062393
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
