tools/xenstore: use treewalk for creating node records

tools/xenstore: use treewalk for creating node records

Instead of doing an open tree walk using call recursion, use
walk_node_tree() when creating the node records during a live update.

This will reduce code size and avoid many nesting levels of function
calls which could potentially exhaust the stack.

This is part of XSA-418 / CVE-2022-42321.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>