AioContext: run bottom halves after polling

Make the dispatching phase the same before blocking and afterwards.
The next patch will make aio_dispatch public and use it directly
for the GSource case, instead of aio_poll.  aio_poll can then be
simplified heavily.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

aio-posix.c

@@ -264,6 +264,10 @@ bool aio_poll(AioContext *ctx, bool blocking)
 
     /* Run dispatch even if there were no readable fds to run timers */
     aio_set_dispatching(ctx, true);
+    if (aio_bh_poll(ctx)) {
+        progress = true;
+    }
+
     if (aio_dispatch(ctx)) {
         progress = true;
     }

aio-win32.c

@@ -143,7 +143,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
 {
     AioHandler *node;
     HANDLE events[MAXIMUM_WAIT_OBJECTS + 1];
-    bool progress;
+    bool progress, first;
     int count;
     int timeout;
 
@@ -177,6 +177,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
     }
 
     ctx->walking_handlers--;
+    first = true;
 
     /* wait until next event */
     while (count > 0) {
@@ -186,6 +187,11 @@ bool aio_poll(AioContext *ctx, bool blocking)
             ? qemu_timeout_ns_to_ms(aio_compute_timeout(ctx)) : 0;
         ret = WaitForMultipleObjects(count, events, FALSE, timeout);
 
+        if (first && aio_bh_poll(ctx)) {
+            progress = true;
+        }
+        first = false;
+
         /* if we have any signaled events, dispatch event */
         if ((DWORD) (ret - WAIT_OBJECT_0) >= count) {
             break;