aio-posix: fix test-aio /aio/event/wait with fdmon-io_uring

aio-posix: fix test-aio /aio/event/wait with fdmon-io_uring

When a file descriptor becomes ready we must re-arm POLL_ADD.  This is
done by adding an sqe to the io_uring sq ring.  The ->need_wait()
function wasn't taking pending sqes into account and therefore
io_uring_submit_and_wait() was not being called.  Polling for cqes
failed to detect fd readiness since we hadn't submitted the sqe to
io_uring.

This patch fixes the following tests/test-aio -p /aio/event/wait
failure:

  ok 11 /aio/event/wait
  **
  ERROR:tests/test-aio.c:374:test_flush_event_notifier: assertion failed: (aio_poll(ctx, false))

Reported-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Tested-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20200402145434.99349-1-stefanha@redhat.com
Fixes: 73fd282e7b6dd4e4ea1c3bbb3d302c8db51e4ccf
       ("aio-posix: add io_uring fd monitoring implementation")
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>