conf: domain: Simplify return from backing store parser

Use VIR_STEAL_PTR to remove conditional cleanup.

src/conf/domain_conf.c

@@ -8327,11 +8327,10 @@ virDomainDiskBackingStoreParse(xmlXPathContextPtr ctxt,
         virDomainDiskBackingStoreParse(ctxt, backingStore, flags) < 0)
         goto cleanup;
 
-    src->backingStore = backingStore;
+    VIR_STEAL_PTR(src->backingStore, backingStore);
     ret = 0;
 
  cleanup:
-    if (ret < 0)
     virStorageSourceFree(backingStore);
     VIR_FREE(type);
     VIR_FREE(format);