file size info: prepare to make format parameter mandatory

In almost all cases, there already is an expected format. Allow
setting a special value 'auto-detect' to opt-in to automatic format
detection by 'qemu-img' for the exceptions.

Suggested-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>

src/PVE/Storage.pm

@@ -2223,7 +2223,7 @@ sub assert_iso_content {
     my ($path) = @_;
 
     # check for things like backing image
-    file_size_info($path, undef, undef, 1);
+    file_size_info($path, undef, 'auto-detect', 1);
 
     return 1;
 }