Use same algorithm as in libblkid from util-linux v2.30.
1. Take first 16 bytes from UTF-8 encoded string of VolumeSetIdentifier
2. If all bytes are hexadecimal digits, convert to lowercase and use as UUID
3. If first 8 bytes are not all hexadecimal digits, convert those 8 bytes
to their
UDF dstring has stored length in the last byte of buffer. Therefore last
byte is not part of recorded characters. And empty string in dstring is
encoded as empty buffer, including first byte (compression id).
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I'm not sure how Grub2 should handle empty label. Current patch set
empty buffer for
