efitable: Don't assume EFI GUID are uuid_t

To share code better with EDK2, we need to align our interfaces with its
definition of EFI_GUID. This structure is the same size as our uuid_t,
but has different elements. The EFI standard uses a string notaiton that
are compatible with the uuid_* routines, however. So we use those
routies to parse, but need to b careful to cast as appropriate. Becuas
the ABI is the same, this is safe, though perhaps suboptimal. These
changes make efitable API neutral.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D50036