Comment 2 for bug 1848535

This is a bit of whack-a-mole game with ptable values that we probe. I plan to omit included probed values that curtin cannot use; 'mac' and 'PMBR' are present on the source disk, but curtin will never create partition tables of these types. When we omit a ptable entry on a disk, subiquity (or curtin) will provide a valid default value.