efivar: Add some explicit casts to uuid_t

[imp]

Summary

Since we know we're type-punning the EFI_GUID to a uuid_t, add some
explicit casts even though, at the moment, we typedef one to the other.

Sponsored by: Netflix

[kib]

I would add some type-checking for the conversion. E.g you could define inlines uuid_t * efi_guid_to_uuid(efi_guid_t *) and const uuid_t *const_efi_guid_to_uuid(const efi_guid_t *) and use them instead of manually writing the cast.