virtio-net: don't dma while vm is stopped

DMA into memory while VM is stopped makes it
hard to debug migration (consequitive saves
result in different files).
Fixing this completely is a large effort,
this patch does this for virtio-net.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Jason Wang <jasowang@redhat.com>

hw/virtio-net.c

@@ -424,6 +424,9 @@ static void virtio_net_handle_rx(VirtIODevice *vdev, VirtQueue *vq)
 static int virtio_net_can_receive(VLANClientState *nc)
 {
     VirtIONet *n = DO_UPCAST(NICState, nc, nc)->opaque;
+    if (!n->vm_running) {
+        return 0;
+    }
 
     if (!virtio_queue_ready(n->rx_vq) ||
         !(n->vdev.status & VIRTIO_CONFIG_S_DRIVER_OK))