disk_setup: fix several issues with gpt disk partitions.

This fixes several shortcomings of disk_setup with gpt disks.
 * 'sgdisk -p' was being used to determine the size of a disk.
   this can fail if it believes there is a bad gpt partition table.
   Instead we just use blockdev now for both mbr or gpt disks.
 * parsing of sgdisk -p output assumed that the 'name' of the partition
   type would not have any spaces (Microsoft basic data)
 * interaction with sgdisk did not realize that sgdisk wants input
   of '8300' rather than '83' and will output the same.

LP: #1692087

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