efi: Use EDK2 EFI_GUID instead of sturct uuid

So, while struct uuid and EFI_GUID are quite similar (they are the same
size, they have the same binary representation, etc), they aren not
identical. Speciifcally, the UUID and GUID defines are slightly
different and can't be swapped one for the other. Start to use EFI_GUID
for all EFI tables. This will allow increased sharing with the boot
laoder that already uses EDK2 variants as well as using the possibility
to use the EDK2 headers we've already imported more easily.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D50037