feat: add start command (#72)

[mprey]

Closes #72

Notes:

• HyperKit cannot support a start command as we are only managing the process PID to control the state of the VM
• start on HyperKit would be the equivalent of run, but since commands/run.js has extra functionality, throw an error instead of trying to duplicate code in commands/start.js
• KVM start will not work after slim stop <vm> -p kvm since this uses destroy instead of shutdown. See Providing acpid support for KVM #75 to see remedy. KVM start will only work after unexpected shutdown

[ssmirr]

Thanks for the PR @mprey! 👍

This looks good, but lets make a decision in #75 before we merge this?