aio-posix: support userspace polling of fd monitoring

Unlike ppoll(2) and epoll(7), Linux io_uring completions can be polled
from userspace.  Previously userspace polling was only allowed when all
AioHandler's had an ->io_poll() callback.  This prevented starvation of
fds by userspace pollable handlers.

Add the FDMonOps->need_wait() callback that enables userspace polling
even when some AioHandlers lack ->io_poll().

For example, it's now possible to do userspace polling when a TCP/IP
socket is monitored thanks to Linux io_uring.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20200305170806.1313245-7-stefanha@redhat.com
Message-Id: <20200305170806.1313245-7-stefanha@redhat.com>

util/fdmon-poll.c

@@ -103,4 +103,5 @@ static void fdmon_poll_update(AioContext *ctx,
 const FDMonOps fdmon_poll_ops = {
     .update = fdmon_poll_update,
     .wait = fdmon_poll_wait,
+    .need_wait = aio_poll_disabled,
 };