Fix release of outgoing stream confirmation/abort message

Fix release of outgoing stream confirmation/abort message

When sending back the final OK or ERROR message on completion
of a stream, we were not decrementing the 'nrequests' tracker
on the client. With the default requests limit of '5', this
meant once a client had created 5 streams, they are unable to
process any further RPC calls.  There was also a bug when
handling an error from decoding a message length header, which
meant a client connection would not immediately be closed.

* src/rpc/virnetserverclient.c: Fix release of request after
  stream completion & mark client for close on error