gtk: Fix accelerator filtering

This is in fact very simply: When the input in grabbed, everything
should be exclusively passed to the guest - except it has our magic
CTRL-ALT modifier set. Then let GTK filter out those accels that are in
use. When checking the modifier state, we just need to filter out NUM
and CAPS lock.

Note: Filtering based on hard-coded modifiers breaks overriding
accelerators. Needs to be fixed at a later point.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
