Build bootable VM images

This branch adds vm/desktop-vm-image-x86_64.bst to the repository, which creates a bootable image of latest GNOME.

This branch depends on landing !316 (merged) first.

This will fix #7 (closed)

State of the image

Just some notes here on the current state of the image, it's not perfect but certainly I think it is enough as an initial commit, and we should improve the integration details and figure out CI as next steps.

• Image size is down to 5G, still a bit chunky for my liking but it is what it is
• Checking out the image gives you a ./run-in-qemu.sh script which should "just work" on an x86_64 host with qemu installed.
• PAM configuration seems functional for all of the cases I was able to test
• System boots to gdm automatically
• First boot will cause gnome-initial-setup to run
• Networking should just work automatically
• Geoclue works, as demonstrated by gnome-initial-setup automatically guessing your location
• Handoff from gnome-initial-setup to the first user session works seamlessly, the gnome-getting-started docs then show up in the UI
• Subsequent boots get you to the regular gdm login session
  • Previously created user correctly appears as the only user on the system
  • Login to the user session works