virtio-gpu: tweak scanout disable.

- Factor out the code to virtio_gpu_disable_scanout().
- Allow disable scanout 0, show a message then.
- Clear scanout->resource_id.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20180702162443.16796-2-kraxel@redhat.com