vm-configurator: Disable S3/S4 states for new domains

There seems to be some issues with S3 and S4 power state support in
libvirt/qemu. Lets disable these for now in new domains.

This patch includes some hacks to not bump libvirt-gconfig dependency.
If boxes is built against older libvirt-gconfg, the newly added code
simply compiles to NOOP.

https://bugzilla.gnome.org/show_bug.cgi?id=685544