firmware: MUST boot in HS mode if virtualization is implemented

[andreiw]

This allows a VM implementation but on a host with virtualization support you shouldn't have surprises.

[andreiw]

BRS has this text in the non-normative portion: "UEFI implementations run in 64-bit S-Mode, VS-Mode or HS-Mode, depending on whether virtualization is supported or used."

Again, it's probably a good idea to make this a stronger req.

[jones-drew]

I agree we should strengthen this. Since we've decided not to consider VM targets for this spec (at least for now [1]), then we can avoid the complications of deciding whether or not we require nested virt support and just focus on S-mode vs. HS-mode. The server spec should mandate HS-mode.

[1] riscv-non-isa/server-soc#45 (comment)