plugin: use relative path for qcow2 rebase command

otherwise the resulting qcow2 file will contain an absolute path, which makes
changing the backing path of the directory storage impossible.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
Tested-by: Fiona Ebner <f.ebner@proxmox.com>
Link: https://lore.proxmox.com/20250729115320.579286-4-f.gruenbichler@proxmox.com

src/PVE/Storage/Plugin.pm

@@ -1434,14 +1434,14 @@ sub volume_snapshot_delete {
 
         } else {
             #we rebase the child image on the parent as new backing image
-            my $parentpath = $snapshots->{$parentsnap}->{file};
             print
                 "$volname: deleting snapshot '$snap' by rebasing '$childsnap' on top of '$parentsnap'\n";
+            my $rel_parent_path = get_snap_name($class, $volname, $parentsnap);
             $cmd = [
                 '/usr/bin/qemu-img',
                 'rebase',
                 '-b',
-                $parentpath,
+                $rel_parent_path,
                 '-F',
                 'qcow2',
                 '-f',