Document how to install in a VM from USB image

[Vincent Legoll]

Here is a patch attempting to help newbies like me to try
GuixSD without the burden of installing guix on their current
operating system.

This explains how to use the USB image to boot a VM and be able
to install GuixSD in a disk image.

Please review, any comments appreciated.

This is my first texinfo edition.
This is my first try at GuixSD.

So please be gentle...