ahci: add support for restarting non-queued commands

ahci: add support for restarting non-queued commands

This is easy, since start_dma already restarts processing from the
beginning of the PRDT.

Migration is also easy to cover; the comment about busy_slot is
wrong, busy_slot will only be set if there is an error.  In this
case we have nothing to do really.  The core IDE code will restart
the operation and command list processing will proceed after the
erroring command has been completed.

Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: John Snow <[email protected]>
Message-id: 1424708286[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>