virtio_blk: v1.0 support

Based on patch by Cornelia Huck.

Note: for consistency, and to avoid sparse errors,
      convert all fields, even those no longer in use
      for virtio v1.0.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>