fix #6441: btrfs: fix cloning templates with non-raw disk images

Cloning a template with a disk image that is neither raw nor a
subvolume, for example a qcow2 image, failed on a BTRFS storage with:

  clone failed: Undefined subroutine &PVE::Storage::DirPlugin::clone_image called

For such images, the BTRFS plugin forwards the call to the DirPlugin.
But the DirPlugin does not implement clone_image itself, it inherits it
from the base plugin, and calling it as a plain function does not go
through the inheritance.

Call the implementation of the base plugin directly instead, like it is
already done for create_base.

Fixes: af50c2e ("add BTRFS storage plugin")
Signed-off-by: Michal Fox <me@dualfroz.com>

src/PVE/Storage/BTRFSPlugin.pm

@@ -291,7 +291,7 @@ sub clone_image {
     # If we're not working with a 'raw' file, which is the only thing that's "different" for btrfs,
     # or a subvolume, we forward to the DirPlugin
     if ($format ne 'raw' && $format ne 'subvol') {
-        return PVE::Storage::DirPlugin::clone_image(@_);
+        return PVE::Storage::Plugin::clone_image(@_);
     }
 
     my $imagedir = $class->get_subdir($scfg, 'images');