Add initial support for building bootable VM images
The primary purpose of the Freedesktop SDK is to power application containers, but with the addition of a bootloader and kernel we can also use it to boot virtual machines.
The VM images are intended for testing purposes only -- it's not a goal to produce something aimed at end-users. However it can be very useful for developers to test their changes to system components in a fully integrated VM image, and this approach is more reproducible and reliable than testing system components in a manually hacked-up development environment. This mechanism could also serve as a basis for automated testing like for example GNOME Continuous.
There is currently no init system in the VM image, instead it boots directly into a shell
The ultimate goal here is to build on this functionality to produce bootable VM images of GNOME using BuildStream, using the Freedesktop SDK as a base.