'Check if the given disk type in extra_spec is valid.
:param extra_spec_disk_type: disk type to check
:return: True if valid'

@staticmethod
def is_valid(extra_spec_disk_type):

    return (extra_spec_disk_type in [ImageDiskType.PREALLOCATED, ImageDiskType.SPARSE, ImageDiskType.STREAM_OPTIMIZED, ImageDiskType.THIN])
