Allows guest to receive mouse buttons 4 and 5, aka "SIDE" button
and "EXTRA" button

Signed-off-by: Darrell Walisser <darrell.walisser@gmail.com>
Message-id: GNUALCfDBqhCUvLwBNJaKqxcPewMtlqCnixk8xTrgI@ubuntu
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>