Add API for Game/CGame to load [legacy] VMs

[zturtleman]

SoF2 has separate gametype QVMs, which could be handled by loading through Game VM.

It would also be useful for making a VM that acts as a compatibility layer for legacy VMs.

syscall to load VM (by name). returns handle.
syscall to call vmMain in VM (by handle) with args.

whenever VM makes a syscall, route cmd/args to VM that loaded it.