tools/xenstore: don't let special watches be children of /

tools/xenstore: don't let special watches be children of /

When firing special watches (e.g. "@releaseDomain"), they will be
regarded to be valid children of the "/" node. So a domain having
registered a watch for "/" and having the privilege to receive
the special watches will receive those special watch events for the
registered "/" watch.

Fix that by calling the related fire_watches() with the "exact"
parameter set to true, causing a mismatch for the "/" node.

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