qemuDomainSnapshotDiskPrepareOne: Fix logic of relative backing store update

Commit 2ace7a87 introduced a logic bug by an improperly
modified condition where we'd skip to the else branch when reusing of
external images was requested and blockdev is available.

The original intentions were to skip the backing store update with
blockdev.

Fix it by only asserting the boolean which was used to track whether we
support update of the backing store only when blockdev is not present
along with the appropriate rename.

https://bugzilla.redhat.com/show_bug.cgi?id=1820016Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

src/qemu/qemu_driver.c

@@ -15129,7 +15129,7 @@ qemuDomainSnapshotDiskPrepareOne(virQEMUDriverPtr driver,
 {
     virDomainDiskDefPtr persistdisk;
     bool supportsCreate;
-    bool supportsBacking;
+    bool updateRelativeBacking = false;
 
     dd->disk = disk;
 
@@ -15158,19 +15158,22 @@ qemuDomainSnapshotDiskPrepareOne(virQEMUDriverPtr driver,
     }
 
     supportsCreate = virStorageFileSupportsCreate(dd->src);
-    supportsBacking = virStorageFileSupportsBackingChainTraversal(dd->src);
 
-    if (supportsCreate || supportsBacking) {
+    /* relative backing store paths need to be updated so that relative
+     * block commit still works. With blockdev we must update it when doing
+     * commit anyways so it's skipped here */
+    if (!blockdev &&
+        virStorageFileSupportsBackingChainTraversal(dd->src))
+        updateRelativeBacking = true;
+
+    if (supportsCreate || updateRelativeBacking) {
         if (qemuDomainStorageFileInit(driver, vm, dd->src, NULL) < 0)
             return -1;
 
         dd->initialized = true;
 
-        /* relative backing store paths need to be updated so that relative
-         * block commit still works. With blockdev we must update it when doing
-         * commit anyways so it's skipped here */
-        if (reuse && !blockdev) {
-            if (supportsBacking) {
+        if (reuse) {
+            if (updateRelativeBacking) {
                 g_autofree char *backingStoreStr = NULL;
 
                 if (virStorageFileGetBackingStoreStr(dd->src, &backingStoreStr) < 0)