qtest/ahci: Add PIO and LBA48 tests

qtest/ahci: Add PIO and LBA48 tests

In addition to DMA tests, test PIO and LBA48 command pathways in AHCI.
To accomplish this, a primitive multiplexer for gtest is added.

Though guests may prefer not to issue PIO commands directly except
for single sector cases during early boot and shutdown, these pathways
are still used for the transfer of ATAPI commands as well, and should
be behaving well.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1424905602-24715-6-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>