Fix reporting of cert validation failures

Fix reporting of cert validation failures

If the server succesfully validates the client cert, it will send
back a single byte, under TLS. If it fails, it will close the
connection. In this case, we were just reporting the standard
I/O error. The original RPC code had a special case hack for the
GNUTLS_E_UNEXPECTED_PACKET_LENGTH error code to make us report
a more useful error message

* src/rpc/virnetclient.c: Return ENOMSG if we get
  GNUTLS_E_UNEXPECTED_PACKET_LENGTH
* src/rpc/virnettlscontext.c: Report cert failure if we
  see ENOMSG