Low: fencing: always add notification type to client notification data

fenced_send_notification() sets F_STONITH_OPERATION to the notification subtype
(T_STONITH_NOTIFY_FENCE, etc.) in notification XML.

Previously, the fencing library would add this as stonith_event_t:operation
only for fencing action notifications. Now, add it for all notification
subtypes (including, for example, device and topology level changes).