Runners not starting when host has no display

[josh-burton]

What happened?

I've got tartelet configured on a mac mini that does not have a display connected. I VNC into the machine.

I've observed that often the VMs are unable to start again after a job has completed. As soon as I VNC into the machine the VMs boot and carry on. This seems to point to an issue where a display is required for the VM to boot.

What are the steps to reproduce?

As above

What is the expected behavior?

VMs should be able to boot if no display is connected.

Potentially support running the tart VMs with the --no-graphics or --vnc option.