qtest/ahci: Add DMA test variants

qtest/ahci: Add DMA test variants

These test a few different pathways in the AHCI code.

short:  Test the minimum transfer size, exactly one sector.
simple: Test a transfer using a single PRD, in this case, 4K.
double: Test transferring 8K, which we will split up as two PRDs.
long:   Test transferring a lot of data using many PRDs, 256K.
Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1424905602-24715-5-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>