Fix release of virNetMessagePtr instances in streams processing

Fix release of virNetMessagePtr instances in streams processing

If a message packet for a invalid stream is received it is just
free'd. This is not good because it doesn't let the client RPC
request counter decrement. If a stream is shutdown with pending
packets the message also isn't released properly because of an
incorrect header type

* daemon/stream.c: Fix message header type
* src/rpc/virnetserverprogram.c: Send dummy reply instead of
  free'ing ignored stream message