parttypes can be either:
- A code for MBR/DOS
- An UUID for GPT
User needs a label instance before using a parttype getter. If checking
for a parttype using a code, check if the current label support that
kind of parttype identification before retrieving it. Same applies for
UUID based parttypes.
If wrong identity type is used, raise an exception. Errors regarding
parttype are more informative this way.
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