ahci: Add test_hba_enable to ahci-test.

This test engages the HBA functionality and initializes
values to sane defaults to allow for minimal HBA functionality.

Buffers are allocated and pointers are updated to allow minimal
I/O commands to complete as expected. Error registers and responses
are sanity checked for specification adherence.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1408643079-30675-8-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
