libblkid: udf: Change algorithm for reporting UUID

libblkid: udf: Change algorithm for reporting UUID

Ensure that reported UUID always contains only lowercase hexadecimal digits
and is always 16 characters length, padded with zero digits.

Volume Set Identifier is converted to UTF-8 before generating UUID from it.
As it could potentially contain any Unicode character. So correctly handle
both 8bit and 16bit OSTA Compressed Unicode encodings.

Disks which have only lowercase hexadecimal digits in Volume Set Identifier
would have same UUID as before this patch.