igvmfilegen: the reset vectors built for vp start context are written in raw opcodes

[babayet2]

Filing this for the backlog on request from @chris-oo

Updating reset vector code in igvmfilegen requires writing raw opcodes, which can lead to bugs like #2341; It would be better if we exposed methods for the programmer to write asm instead

[smalis-msft]

We could maybe use iced for this, like what we do for the x86_emu tests.