notify io_thread at the end of rx handling

This is a backport from qemu-kvm. Just instead of using kvm's specific
notification mechanism, we use qemu_notify_event()

Signed-off-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/virtio-net.c

@@ -305,6 +305,10 @@ static void virtio_net_handle_rx(VirtIODevice *vdev, VirtQueue *vq)
     VirtIONet *n = to_virtio_net(vdev);
 
     qemu_flush_queued_packets(n->vc);
+
+    /* We now have RX buffers, signal to the IO thread to break out of the
+     * select to re-poll the tap file descriptor */
+    qemu_notify_event();
 }
 
 static int do_virtio_net_can_receive(VirtIONet *n, int bufsize)