qemu: domain: update only newly detected images in qemuDomainDetermineDiskChain

The processing code which prepares images should be executed really only
for the images which were detected. The code actually tried to update
the last user-specified layer as well. Thankfully we don't do anything
that would be a problem at this point.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>

src/qemu/qemu_domain.c

@@ -8403,7 +8403,7 @@ qemuDomainDetermineDiskChain(virQEMUDriverPtr driver,
     if (virStorageFileGetMetadata(src, uid, gid, report_broken) < 0)
         goto cleanup;
 
-    for (n = src; virStorageSourceIsBacking(n); n = n->backingStore) {
+    for (n = src->backingStore; virStorageSourceIsBacking(n); n = n->backingStore) {
         if (qemuDomainValidateStorageSource(n, priv->qemuCaps) < 0)
             goto cleanup;