blktap: Add fallback code to blktap for missing poll-on-aio support.

blktap requires a xen specific kernel AIO ABI which has been vetoed by
upstream in favour of another approach. Rather than include this ABI,
Fedora has been carrying a patch which makes tap:aio use a thread to
poll for aio events and notify the main thread via a pipe.

The upstream approach of allowing io_getevents() poll normal file
descriptors via epoll is still progressing:
 http://lkml.org/lkml/2007/1/3/16

but when that does make it upstream, blktap will require significant
re-working to use that approach.

In the meantime, here's a patch which uses the poll-in-a-thread
approach only if AIO poll support isn't available. It also hides the
details behind a simple abstraction and makes both tap:aio and
tap:qcow use it.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>

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