aio: make aio_poll(ctx, true) block with no fds

This patch drops a special case where aio_poll(ctx, true) returns false
instead of blocking if no file descriptors are waiting on I/O.  Now it
is possible to block in aio_poll() to wait for aio_notify().

This change eliminates busy waiting.  bdrv_drain_all() used to rely on
busy waiting to completed throttled I/O requests but this is no longer
required so we can simplify aio_poll().

Note that aio_poll() still returns false when aio_notify() was used.  In
other words, stopping a blocking aio_poll() wait is not considered
making progress.

Adjust test-aio /aio/bh/callback-delete/one which assumed aio_poll(ctx,
true) would immediately return false instead of blocking.

Reviewed-by: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

tests/test-aio.c

@@ -195,7 +195,6 @@ static void test_bh_delete_from_cb(void)
     g_assert(data1.bh == NULL);
 
     g_assert(!aio_poll(ctx, false));
-    g_assert(!aio_poll(ctx, true));
 }
 
 static void test_bh_delete_from_cb_many(void)