hack(cgpt): use Root instead of the Kernel partitions

hack up the code a bit to set priorities on root partitions:

$ cgpt show -i 4 -P /dev/sda
1
$ cgpt show -i 5 -P /dev/sda
2

$ cgpt prioritize -i 4 /dev/sda
$ cgpt show -i 4 -P /dev/sda
2