Adds command header helper functions:
-ahci_command_header_set
-ahci_command_header_get,
-ahci_command_destroy, and
-ahci_cmd_pick

These helpers help to quickly manage the command header information in
the AHCI device.

ahci_command_header_set and get will store or retrieve an AHCI command
header, respectively.

ahci_cmd_pick chooses the first available but least recently used
command slot to allow us to cycle through the available command slots.

ahci_command_destroy obliterates all information contained within a
given slot's command header, and frees its associated command table,
but not its DMA buffer!

Lastly, the command table pointer fields (dba and dbau) are merged into
a single 64bit value to make managing 64bit tests simpler.
Signed-off-by: NJohn Snow <jsnow@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-id: 1423158090-25580-5-git-send-email-jsnow@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>