Provide a vmctl start|stop|exec command

This wraps the common things we need to do - start the vm, stop it later and in between exec a few commands on the VM itself.

exec is just a wrapper around ssh and vmctl start installs a ssh_config.d entry so we can acces the vm as host "vm". This simplifies our scp calls a lot too since we don't need to know about the port anymore and calling it "vm" is less confusing than "localhost".

FTR, I did have a vmctl copy command in the test branch but it ended up being worse than just calling scp - the argument for exec is weak enough as it is.