xenstore: modify add_change_node() parameter types

xenstore: modify add_change_node() parameter types

In order to prepare adding a generation count to each node modify
add_change_node() to take the connection pointer and a node pointer
instead of the transaction pointer and node name as parameters. This
requires moving the call of add_change_node() from do_rm() to
delete_node_single().

While at it correct the comment for the prototype: there is no
longjmp() involved.

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