rpc: Refactor the condition whether a client needs authentication

rpc: Refactor the condition whether a client needs authentication

Add virNetServerClientAuthMethodImpliesAuthenticated() for deciding
whether a authentication method implies that a client is automatically
authenticated or not. Use this new function in
virNetServerClientNeedAuthLocked().

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>