spice: wakeup QXL worker to pick up mouse changes

Without it, server-mode mouse is "slow" to update position: QXL will
wait until new display commands come. This is very visible with
virtio-gpu.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20170130104540.14660-1-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>