libvirt-machine-props: Restart only sensitive for running machine
libvirt-machine-props: Restart only sensitive for running machine

If machine in question is not running, users shouldn't be clicking on
"Restart" button, just like it's the case for "Force shutdown" button.
Let's make this button's sensitivity be dependent on machine's status.

vbenes/gnome-boxes,vbenes/gnome-boxes