virtio_blk: SG_IO passthru support

virtio_blk: SG_IO passthru support

Add support for SG_IO passthru to virtio_blk.  We add the scsi command
block after the normal outhdr, and the scsi inhdr with full status
information aswell as the sense buffer before the regular inhdr.

[hch: forward ported, added the VIRTIO_BLK_F_SCSI flags, some comments
 and tested the whole beast]
[axboe: updated to use ->resid and not dual-path the byte count]

Signed-off-by: Hannes Reinecke <[email protected]>
Signed-off-by: Christoph Hellwig <[email protected]>
Signed-off-by: Rusty Russell <[email protected]> (+ checkpatch.pl tweak)
Signed-off-by: Jens Axboe <[email protected]>