rpc: Replace virNetServerClientNeedAuth with virNetServerClientIsAuthenticated
    
    Replace virNetServerClientNeedAuth with
    virNetServerClientIsAuthenticated because it makes it clearer what it
    means.
    
    Signed-off-by: Marc Hartmayer <mhartmay@linux.vnet.ibm.com>
    Reviewed-by: Boris Fiuczynski <fiuczy@linux.vnet.ibm.com>
    Reviewed-by: Stefan Zimmermann <stzi@linux.vnet.ibm.com>