Have the vif scripts expect the offline event, not the remove event. With

Have the vif scripts expect the offline event, not the remove event.  With
the remove event, the script environment variable has disappeared, so we must
hook on the offline event instead.  This fix was put into xen-backend.rules,
but not the vif scripts, so they were out of sync, and the interfaces were not
being properly closed down.

Signed-off-by: Ewan Mellor <ewan@xensource.com>