ahci: add cmd header to ncq transfer state

ahci: add cmd header to ncq transfer state

While the rest of the AHCI device can rely on a single bookmarked
pointer for the AHCI Command Header currently being processed, NCQ
is asynchronous and may have many commands in flight simultaneously.

Add a cmdh pointer to the ncq_tfs object and make the sglist prepare
function take an AHCICmdHeader pointer so we can be explicit about
where we'd like to build SGlists from.

Signed-off-by: John Snow <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-id: 1435767578[email protected]