qemu: block: Don't lookup node names if they are already known

Move the check that skips node name detection if they are already
present earlier so that the hash table lookup is skipped.

src/qemu/qemu_block.c

@@ -272,13 +272,13 @@ qemuBlockDiskDetectNodes(virDomainDiskDefPtr disk,
     qemuBlockNodeNameBackingChainDataPtr entry = NULL;
     virStorageSourcePtr src = disk->src;
 
-    if (!(entry = virHashLookup(disktable, disk->info.alias)))
-        return 0;
-
     /* don't attempt the detection if the top level already has node names */
     if (src->nodeformat || src->nodestorage)
         return 0;
 
+    if (!(entry = virHashLookup(disktable, disk->info.alias)))
+        return 0;
+
     while (src && entry) {
         if (src->nodeformat || src->nodestorage) {
             if (STRNEQ_NULLABLE(src->nodeformat, entry->nodeformat) ||