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

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.
Drop the aio_flush() test case.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>