qemu: block: Use correct alias when extracting disk node names

The alias recorded in disk->info.alias is the alias for the frontend
device but we are interested in the backend drive. This messed up the
disk node name extraction code as qemu reports the drive alias in the
block query commands. This was broken in the node name detector
refactoring done in commit 0175dc6e

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1494327

src/qemu/qemu_block.c

@@ -20,6 +20,7 @@
 
 #include "qemu_block.h"
 #include "qemu_domain.h"
+#include "qemu_alias.h"
 
 #include "viralloc.h"
 #include "virstring.h"
@@ -271,36 +272,46 @@ qemuBlockDiskDetectNodes(virDomainDiskDefPtr disk,
 {
     qemuBlockNodeNameBackingChainDataPtr entry = NULL;
     virStorageSourcePtr src = disk->src;
+    char *alias = NULL;
+    int ret = -1;
 
     /* 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;
+    if (!(alias = qemuAliasFromDisk(disk)))
+        goto cleanup;
+
+    if (!(entry = virHashLookup(disktable, alias))) {
+        ret = 0;
+        goto cleanup;
+    }
 
     while (src && entry) {
         if (src->nodeformat || src->nodestorage) {
             if (STRNEQ_NULLABLE(src->nodeformat, entry->nodeformat) ||
                 STRNEQ_NULLABLE(src->nodestorage, entry->nodestorage))
-                goto error;
+                goto cleanup;
 
             break;
         } else {
             if (VIR_STRDUP(src->nodeformat, entry->nodeformat) < 0 ||
                 VIR_STRDUP(src->nodestorage, entry->nodestorage) < 0)
-                goto error;
+                goto cleanup;
         }
 
         entry = entry->backing;
         src = src->backingStore;
     }
 
-    return 0;
+    ret = 0;
 
- error:
-    qemuBlockDiskClearDetectedNodes(disk);
-    return -1;
+ cleanup:
+    VIR_FREE(alias);
+    if (ret < 0)
+        qemuBlockDiskClearDetectedNodes(disk);
+
+    return ret;
 }