Comment 1 for bug 2037566

Proposal of a new machine constraint:

   nested-virtualization=[true|false]

My expectation is that one would want to add machines in a model, some with this enabled, and others without. This would facilitate something like Charmed Kubernetes with kubevirt enabled. The worker machines in the model could be started with a constraint like `nested-virtualization=true' which would allow `/dev/kvm` from the metal to be passed through and used to start VMs inside the workers without adding `/dev/kvm` the machines that host kubernetes-control-plane, etcd, vault, kubeapi-load-balancer and other Charmed Kubernetes machines