xenstore: call add_change_node() directly when writing node

xenstore: call add_change_node() directly when writing node

Instead of calling add_change_node() at places where write_node() is
called, do that inside write_node().

Note that there is one case where add_change_node() is called now when
a later failure will prohibit the changed node to be written: in case
of a write_node failing due to an error in tdb_store(). As the only
visible change of behavior is a stale event fired for the node, while
the failing tdb_store() signals a corrupted xenstore database, the
stale event will be the least problem of this case.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>