NCQ commands have the concept of a "TAG" that they need to set,
but in the AHCI world, it is mandated that the TAG always match
the command slot that you executed the NCQ from.

See AHCI 9.3.1.1.5.2 "Native Queued Commands".

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1435016308-6150-14-git-send-email-jsnow@redhat.com