tools/xenstore: let unread watch events time out

tools/xenstore: let unread watch events time out

A future modification will limit the number of outstanding requests
for a domain, where "outstanding" means that the response of the
request or any resulting watch event hasn't been consumed yet.

In order to avoid a malicious guest being capable to block other guests
by not reading watch events, add a timeout for watch events. In case a
watch event hasn't been consumed after this timeout, it is being
deleted. Set the default timeout to 20 seconds (a random value being
not too high).

In order to support to specify other timeout values in future, use a
generic command line option for that purpose:

--timeout|-w watch-event=<seconds>

This is part of XSA-326 / CVE-2022-42311.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
(cherry picked from commit 5285dcb1a5c01695c11e6397c95d906b5e765c98)