virtio-blk: don't let virtio core to validate used length

We never tries to use used length, so the patch prevents the virtio
core from validating used length.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20211027022107.14357-4-jasowang@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>