tools/xenstore: move the call of setup_structure() to dom0 introduction

tools/xenstore: move the call of setup_structure() to dom0 introduction

Setting up the basic structure when introducing dom0 has the advantage
to be able to add proper node memory accounting for the added nodes
later.

This makes it possible to do proper node accounting, too.

An additional requirement to make that work fine is to correct the
owner of the created nodes to be dom0_domid instead of domid 0.

This is part of XSA-326.

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