Change `bcvk libvirt ssh` to auto-start stopped VMs

[cgwalters]

This would pair well with integrated support for auto-stop on idle; we could do that by injecting a systemd unit for it.

[cgwalters]

we could do that by injecting a systemd unit for it.

Note we have an ongoing challenge in that some use cases (devcontainers and things like that) don't run systemd by default.