make domain_create() return a proper error code

make domain_create() return a proper error code

While triggered by the XSA-9 fix, this really is of more general use;
that fix just pointed out very sharply that the current situation
with all domain creation failures reported to user (tools) space as
-ENOMEM is very unfortunate (actively misleading users _and_ support
personnel).

Pull over the pointer <-> error code conversion infrastructure from
Linux, and use it in domain_create() and all it callers.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>