Save the parent object's open function pointer in the (new)
VCChardevClass struct instead before overwriting it, so we
can look it up when needed.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20201014121120.13482-3-kraxel@redhat.com