ide/atapi: make PIO read requests async

PIO read requests on the ATAPI interface used to be sync blk requests.
This has two significant drawbacks. First the main loop hangs util an
I/O request is completed and secondly if the I/O request does not
complete (e.g. due to an unresponsive storage) Qemu hangs completely.

Note: Due to possible race conditions requests during an ongoing
elementary transfer are still sync.

Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 1447345846-15624-2-git-send-email-pl@kamp.de
Signed-off-by: John Snow <jsnow@redhat.com>
