part: rename disk_partition_type_uuid()

Rename disk_partition_type_uuid to disk_partition_type_guid.

Provide function descriptions for the getter and setter.

Fixes: bcd64542 ("part: Add accessors for struct disk_partition type_uuid")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>