Don't return a fatal error if receiving unexpected stream data

Don't return a fatal error if receiving unexpected stream data

Due to the asynchronous nature of streams, we might continue to
receive some stream packets from the server even after we have
shutdown the stream on the client side. These should be discarded
silently, rather than raising an error in the RPC layer.

* src/rpc/virnetclient.c: Discard stream data silently