rpcserver: fix crash in XML-RPC system commands
Fix an AttributeError in XML-RPC methodSignature and methodHelp commands
caused by incorrect mangled name usage.
https://fedorahosted.org/freeipa/ticket/6217
Reviewed-By: Lenka Doudova <ldoudova@redhat.com>
Reviewed-By: David Kupka <dkupka@redhat.com>