hw/block/vhost-user-blk: Use DEVICE() / VIRTIO_DEVICE() macros

Access QOM parent with the proper QOM [VIRTIO_]DEVICE() macros.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20231017140150.44995-3-philmd@linaro.org>