tools/ocaml/xenstored: Synchronise defaults with oxenstore.conf.in

tools/ocaml/xenstored: Synchronise defaults with oxenstore.conf.in

We currently have 2 different set of defaults in upstream Xen git tree:
* defined in the source code, only used if there is no config file
* defined in the oxenstored.conf.in upstream Xen

An oxenstored.conf file is not mandatory, and if missing, maxrequests in
particular has an unsafe default.

Resync the defaults from oxenstored.conf.in into the source code.

This is part of XSA-326 / CVE-2022-42316.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>