[UEFILDR] Revise disk access code (reactos#8623)

This commit reworks how the uefidisk code works so it's not longer... completely wrong.
This also allows GPT disks to now be used on uefildr.
It's not perfect (as the correct solution is having freeldr load full efi device paths instead of arc like bootmgr does)
but this will work perfectly fine now. Well should.

Author: DarkFire01