AHCI: Protect cmd register

AHCI: Protect cmd register

Many bits in the CMD register are supposed to be strictly read-only.
We should not be deleting them on every write.

As a side-effect: pay explicit attention to when a guest marks off
the FIS Receive or Start bits, and disable the status bits ourselves,
instead of letting them implicitly fall off.

Signed-off-by: John Snow <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-id: 1426283454[email protected]