Fix two bugs in the calculation of the last LBA of the GPT covered

Fix two bugs in the calculation of the last LBA of the GPT covered
part of the disk. The first appears to be a typo and instead of
dividing the media size with the sector size, we multiplied. The
second is an off-by-1 error that's the result of mixing up count
and index. The code in question is only applicable for virgin disks
and is used to create the "whole" chunk, which covers only the GPT
usable portion of the disk.