Support starting VMs inside of a workshop container

For developing *craft apps, we need to be able to create both VMs and containers. It'd be nice if we could get an SDK or something that would enable nested containers and [pass through the KVM device](https://discuss.linuxcontainers.org/t/running-vm-s-inside-lxd-containers/14884/7). It's probably worth noting that this would reduce the isolation, so it should only be opt-in (and probably with a warning about it on launch), but it would make end-to-end testing of crafts far nicer.