virtio: Add missing documentation for structure fields

Add missing documentation for the vqs_list_lock field of struct virtio_device,
and the validate field of struct virtio_driver.

./scripts/kernel-doc says:

 .../virtio.h:131: warning: Function parameter or member 'vqs_list_lock' not described in 'virtio_device'
 .../virtio.h:192: warning: Function parameter or member 'validate' not described in 'virtio_driver'
 2 warnings as Errors

No functional changes intended.

Signed-off-by: Simon Horman <horms@kernel.org>
Message-Id: <20230510-virtio-kdoc-v3-1-e2681ed7a425@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>