This adds initial virtio-scsi support. We have no scsi layer in osv, in
this implementation virtio-scsi works directly with the bio layer. It
translates BIO_READ, BIO_WRITE and BIO_FLUSH to SCSI CMD.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>