maint: The 'release' target builds a VM image.

* gnu/system/examples/vm-image.tmpl: New file.
* Makefile.am (GUIXSD_VM_SYSTEMS, GUIXSD_VM_IMAGE_BASE,
GUIXSD_VM_IMAGE_SIZE): New variables.
(release): Add logic to build a VM image.
(EXAMPLES): Add 'gnu/system/examples/vm-image.tmpl'.
* doc/guix.texi (Running GuixSD in a VM, Installing GuixSD in a VM): Mention the
pre-built VM image.