Archipelago » qemu

aio: Fix return value of aio_poll()

aio_poll() must return true if any work is still pending, even if it
didn't make progress, so that bdrv_drain_all() doesn't stop waiting too
early. The possibility of stopping early occasionally lead to a failed
assertion in bdrv_drain_all(), when some in-flight request was missed...

tests: use aio_poll() instead of aio_flush() in test-aio.c

There has been confusion between various aio wait and flush functions.
It's time to get rid of qemu_aio_flush() but in the aio test cases we
really do want this low-level functionality.

Therefore declare a local wait_for_aio() helper for the test cases....