tools/xenstore: simplify and fix per domain node accounting

tools/xenstore: simplify and fix per domain node accounting

The accounting of nodes can be simplified now that each connection
holds the associated domid.

Fix the node accounting to cover nodes created for a domain before it
has been introduced. This requires to react properly to an allocation
failure inside domain_entry_inc() by returning an error code.

Especially in error paths the node accounting has to be fixed in some
cases.

This is part of XSA-326 / CVE-2022-42313.

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