efivar: Allow uuid_t and efi_guid_t to be different types.

While they are binarily the same, the API are different. To use EDK2
more effectively, we need to transition to using EDK@'s API rather than
the uuid_t API. This change makes the code neutral on the subject.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D50035