xenstore: use temporary memory context for firing watches

xenstore: use temporary memory context for firing watches

Use a temporary memory context for memory allocations when firing
watches. This will avoid leaking memory in case of long living
connections and/or xenstore entries.

This requires adding a new parameter to fire_watches() and add_event()
to specify the memory context to use for allocations.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>