libvirt-machine-props: Dismiss props window on restart & shutdown
libvirt-machine-props: Dismiss props window on restart & shutdown

In most cases, users wouldn't want to keep using the properties window
after clicking either restart or force shutdown button. So lets dismiss
the window when user click on these buttons. Also with no other
indication of any progress or activity, this would serve as some feedback
to user that Boxes *is* doing something.

vbenes/gnome-boxes,vbenes/gnome-boxes