vhost_net: init acked_features to backend_features

commit 2e6d46d7 (vhost: add
vhost_get_features and vhost_ack_features) removes the step that
initializes the acked_features to backend_features.

As this field is now uninitialized, vhost initialization will sometimes
fail.

To fix, initialize acked_features on each ack.
Tested-by: NAndrey Korolyov <andrey@xdel.ru>
Cc: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: NJason Wang <jasowang@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
(cherry picked from commit b49ae913)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

hw/net/vhost_net.c

@@ -115,6 +115,7 @@ unsigned vhost_net_get_features(struct vhost_net *net, unsigned features)
 
 void vhost_net_ack_features(struct vhost_net *net, unsigned features)
 {
+    net->dev.acked_features = net->dev.backend_features;
     vhost_ack_features(&net->dev, vhost_net_get_feature_bits(net), features);
 }