libaio: add support for polled IO

libaio: add support for polled IO

The support isn't in mainline yet, but it's being developed.
Currently the kernel support resides in the 'mq-perf' branch
of the block repo:

git://git.kernel.dk/linux-block mq-perf

Also see:

http://git.kernel.dk/cgit/linux-block/log/?h=mq-perf

Signed-off-by: Jens Axboe <axboe@kernel.dk>