disk: part_msdos: parse and store partition UUID

The MSDOS/MBR partition table includes a 32-bit unique ID, often referred
to as the NT disk signature. When combined with a partition number within
the table, this can form a unique ID similar in concept to EFI/GPT's
partition UUID.

This patch generates UUIDs in the format 0002dd75-01, which matches the
format expected by the Linux kernel.

Signed-off-by: Stephen Warren <swarren@nvidia.com>