'Get disk type from the given volume type\'s extra spec.
If there is no disk type option, default is THIN_VMDK_TYPE.'

@staticmethod
def _get_extra_spec_disk_type(type_id):

    disk_type = _get_volume_type_extra_spec(type_id, 'vmdk_type', default_value=THIN_VMDK_TYPE)
    volumeops.VirtualDiskType.validate(disk_type)
    return disk_type
