tools/xenstore: add infrastructure to keep track of per domain memory usage

tools/xenstore: add infrastructure to keep track of per domain memory usage

The amount of memory a domain can consume in Xenstore is limited by
various quota today, but even with sane quota a domain can still
consume rather large memory quantities.

Add the infrastructure for keeping track of the amount of memory a
domain is consuming in Xenstore. Note that this is only the memory a
domain has direct control over, so any internal administration data
needed by Xenstore only is not being accounted for.

There are two quotas defined: a soft quota which will result in a
warning issued via syslog() when it is exceeded, and a hard quota
resulting in a stop of accepting further requests or watch events as
long as the hard quota would be violated by accepting those.

Setting any of those quotas to 0 will disable it.

As default values use 2MB per domain for the soft limit (this basically
covers the allowed case to create 1000 nodes needing 2kB each), and
2.5MB for the hard limit.

This is part of XSA-326.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
(cherry picked from commit 0d4a8ec7a93faedbe54fd197db146de628459e77)