ps2: reset queue in ps2_reset_keyboard

When the guest resets the keyboard also clear the queue.  It is highly
unlikely that the guest is still interested in the events stuck in the
queue, and it avoids confusing the guest in case the queue is full and
the ACK can't be queued up.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1372583
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20170606112105.13331-4-kraxel@redhat.com
