Add ability to run the VM with display device but windowless

[jairov4]

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UTM currently will suspend or shutdown the VM if you press the close button of the guest display window. I have read that you can run headless by removing the display device from the VM, however it is not possible via UI for MacOS guests.

It would be nice to allow setup the VM to run windowless and even better connect to the display using double click and let the guest run in the background when we close the display window.

[wzzrd]

Being able to close the display window without killing the guest would be so great...