virtio: invalidate memory in vring_set_avail_event()

Remember to invalidate the avail event field so the memory pages are
marked dirty.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Eric Auger <eric.auger@redhat.com>

hw/virtio/virtio.c

@@ -282,6 +282,7 @@ static inline void vring_set_avail_event(VirtQueue *vq, uint16_t val)
     caches = atomic_rcu_read(&vq->vring.caches);
     pa = offsetof(VRingUsed, ring[vq->vring.num]);
     virtio_stw_phys_cached(vq->vdev, &caches->used, pa, val);
+    address_space_cache_invalidate(&caches->used, pa, sizeof(val));
 }
 
 void virtio_queue_set_notification(VirtQueue *vq, int enable)