add default mapping of credentials to machine

In the auth config file, it is currently required to have
an entry for each hostname to connect to, eg

  [auth-libvirt-prod1.example.com]
  credentials=prod

This is inconvenient when there are large numbers of machines
all with the same credentials. Add support for a default
entry:

  [auth-default]
  credentials=prod

src/util/virauthconfig.c

@@ -121,6 +121,13 @@ int virAuthConfigLookup(virAuthConfigPtr auth,
     if (virAsprintf(&authgroup, "auth-%s-%s", service, hostname) < 0)
         goto cleanup;
 
+    if (!virKeyFileHasGroup(auth->keyfile, authgroup)) {
+       VIR_FREE(authgroup);
+       if (virAsprintf(&authgroup, "auth-%s-%s", service, "default") < 0){
+           goto cleanup;
+       }
+    }
+
     if (!virKeyFileHasGroup(auth->keyfile, authgroup)) {
         ret = 0;
         goto cleanup;