gtk: Avoid accel key leakage into guest on console switch

GTK2 sends the accel key to the guest when switching to the graphic
console via that shortcut. Resolve this by ignoring any keys until the
next key-release event. However, do not ignore keys when switching via
the menu or when on GTK3.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>