vnc: lift modifier keys on client disconnect.

vnc: lift modifier keys on client disconnect.

For any modifier key (shift, ctrl, alt) still pressed on disconnect
inject a key-up event into the guest.  The vnc client is gone, it will
not do that, so qemu has to do it instead.

Without this keys will get stuck, making the guest act in weird ways
after reconnecting.  Reproducer: exit vnc client via Alt-F4, guest
continues to see the pressed alt key and will not react to key events
in any useful way until you tap the alt key once to unstuck it.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>