ahci: Adding basic functionality qtest.

Currently, there is no qtest to test the functionality of
the AHCI functionality present within the Q35 machine type.

This patch adds a skeleton for an AHCI test suite,
and adds a simple sanity-check test case where we
identify that the AHCI device is present, then
disengage the virtual machine.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1408643079-30675-2-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
