tools/ocaml/libs/xc: Backward compatible domid control at domain creation time

tools/ocaml/libs/xc: Backward compatible domid control at domain creation time

One can specify the domid to use when creating the domain, but this was
hardcoded to 0.

Keep the existing `domain_create` function, and make domid an optional
argument.  When not specified, default to 0.

Controlling the domid can be useful during testing or migration.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>