libqos/ahci: Swap memread/write with bufread/write

libqos/ahci: Swap memread/write with bufread/write

Where it makes sense, use the new faster primitives.
For generally small reads/writes such as for the PRDT
and FIS packets, stick with the more wasteful but
easier to debug memread/memwrite.

For ahci-test (before migration tests):
With this patch:
real    0m3.675s
user    0m2.582s
sys     0m1.718s

Without any qtest protocol improvements:
real    0m14.171s
user    0m12.072s
sys     0m12.527s

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1430864578-22072-6-git-send-email-jsnow@redhat.com