keymap: consider modifier state when picking a mapping

Pass the modifier state to the keymap lookup function.  In case multiple
keysym -> keycode mappings exist look at the modifier state and prefer
the mapping where the modifier state matches.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20180222070513.8740-6-kraxel@redhat.com