ide: Add silent DRQ cancellation

ide: Add silent DRQ cancellation

Split apart the ide_transfer_stop function into two versions: one that
interrupts and one that doesn't. The one that doesn't can be used to
halt any PIO transfers that are in the DRQ phase. It will not halt
any PIO transfers that are currently in the process of buffering data
for the guest to read.

Signed-off-by: John Snow <jsnow@redhat.com>
Reported-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
[Renamed 'etf' to 'end_transfer_func' --js]
Message-id: 1453225191-11871-6-git-send-email-jsnow@redhat.com