Add human-readable command names and other miscellaneous #defines
to help make the code more readable.

Some of these definitions are not yet used in this current series,
but for convenience and sanity they have been lumped together here,
as it's more trouble than it is worth in a test suite to hand-pick,
one-by-one, which preprocessor definitions are useful per-each test.

These definitions include:

ATA Command Mnemonics
Current expected AHCI sector size
FIS magic bytes
REG_H2D_FIS flags
Command Header flags
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Message-id: 1423158090-25580-10-git-send-email-jsnow@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>