Watch the online node in the backend area, as well as the state node

Watch the online node in the backend area, as well as the state node
in the frontend area, and fire the frontend state changed watch
whenever it changes.  This allows us to catch the case where a device
shuts down in a domU and then gets xm detach'd from in dom0.

Otherwise, the backend doesn't shut down correctly, since online was
set when the frontend shut down and we don't get another kick when it
becomes unset.