libqos/ahci: add ahci command helpers

libqos/ahci: add ahci command helpers

ahci_command_set_flags:  Set additional flags in the command header.
ahci_command_clr_flags:  Clear flags from the command header.
ahci_command_set_offset: Change the IO sector from 0.
ahci_command_adjust:     Adjust many values simultaneously.

To be used to adjust the command header if the default values/guesses
were incorrect or undesirable.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1424905602-24715-4-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
[ kwolf: Fixed conflicting prototype for ahci_command_adjust() ]

Signed-off-by: Kevin Wolf <kwolf@redhat.com>