domain_conf: fix domain deadlock

If you use public api virConnectListAllDomains() with second parameter
set to NULL to get only the number of domains you will lock out all
other operations with domains.

Introduced by commit 2c680804.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

src/conf/domain_conf.c

@@ -20560,7 +20560,7 @@ virDomainListPopulate(void *payload,
     /* just count the machines */
     if (!data->domains) {
         data->ndomains++;
-        return;
+        goto cleanup;
     }
 
     if (!(dom = virGetDomain(data->conn, vm->def->name, vm->def->uuid))) {