def get_root_vhd_path(self, instance_name, format_ext):
    instance_path = self.get_instance_dir(instance_name)
    return os.path.join(instance_path, ('root.' + format_ext.lower()))
def get_root_vhd_path(self, instance_name, format_ext):
    instance_path = self.get_instance_dir(instance_name)
    return os.path.join(instance_path, ('root.' + format_ext.lower()))