virtio-blk: What's up with VIRTIO_BLK_T_GET_ID (8)?

[Cornelia Huck <cornelia.huck@de.ibm.com>]

I see the request type in $SUBJECT implemented in both Linux and QEMU,
but I can't find it in the standard.

If we believe that getting a s/n is useful, we should spec it;
otherwise, if we consider it just a legacy request type, it wouldn't
hurt to mention it in the legacy section.