hw/display/virtio-gpu: Use VIRTIO_DEVICE() macro

Access QOM parent with the proper QOM VIRTIO_DEVICE() macro.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20231017140150.44995-4-philmd@linaro.org>