libvirt-machine: More use of new is_on() predicate
libvirt-machine: More use of new is_on() predicate

We don't tell user to reboot the box if machine is exactly in 'stopped'
state. We probably want to apply that to all off states.

https://bugzilla.gnome.org/show_bug.cgi?id=685846

vbenes/gnome-boxes,vbenes/gnome-boxes