I like this plan. The one comment I have is that it seems unnecessarily
opinionated about which operations trigger virtual instruction traps vs
illegal instruction traps when run in VU mode. I think we should error more
on having things trigger virtual instruction traps everywhere that it is
unlikely to require M-mode emulation. To give one example of where the
current design might go wrong: analogously to HLV and friends, HFENCE from
U-mode might be allowed via a CSR in the future, in which case it would now
require hypervisor emulation when a nested VM tried to run it in VU-mode.