ahci: add is_ncq predicate helper

ahci: add is_ncq predicate helper

A small helper to determine which S/ATA commands
are destined to be routed to the NCQ pathways.

This references SATA 3.2 section 13.6,
Native Command Queueing. See sections 13.6.4,
13.6.5, 13.6.6, 13.6.7 and 13.6.8 for all
SATA commands considered to be part of the
NCQ feature set. This is summarized in a small
list in section 13.6.3.1 and again in 13.6.3.2.

Not all of these NCQ commands are currently supported,
so the error pathways are adjusted slightly to be more
informative in the case they are encountered.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1415058979-16604-2-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>