ipops: add dns container in dns_update_pv() if not created before

- dns context creation relied on using the dns variable in some way, the
  main purpose of dns_update_pv() being to fill the variable, but one may
  want to use it for testing only

(cherry picked from commit 543cd8f897bc43d5fe1eb4c562d8351b32778209)

src/modules/ipops/ipops_pv.c

@@ -373,7 +373,7 @@ int dns_update_pv(str *hostname, str *name)
 		return -2;
 	}
 
-	dr = sr_dns_get_item(name);
+	dr = sr_dns_add_item(name);
 	if(dr==NULL)
 	{
 		LM_DBG("container not found: %s\n", name->s);