fix #6969: plugin: keep content directory when freeing its last file

After removing a volume, free_image tries to remove the parent
directory, so that no empty per-guest directories are left behind once
all images of a guest are gone. This was done unconditionally, so
deleting the last ISO, container template, snippet or import file also
removed the shared content directory like 'snippets' or 'template/iso'.

The directory gets re-created shortly after by pvestatd, but as
root:root with default permissions, losing any ownership or mode that
the admin set up, for example to allow a non-root user to upload
snippets.

Only clean up the parent directory for guest images, which are the only
volumes stored in per-guest directories.

Signed-off-by: Michal Fox <me@dualfroz.com>

src/PVE/Storage/Plugin.pm

@@ -1166,9 +1166,9 @@ sub free_image {
     }
 
     # try to cleanup directory to not clutter storage with empty $vmid dirs if
-    # all images from a guest got deleted
-    my $dir = dirname($path);
-    rmdir($dir);
+    # all images from a guest got deleted, but keep the shared content type dirs
+    my ($vtype) = $class->parse_volname($volname);
+    rmdir(dirname($path)) if $vtype eq 'images';
 
     return undef;
 }