domctl.h: document non-standard error codes for enabling paging/access

domctl.h: document non-standard error codes for enabling paging/access

The domctl to enable paging and access returns some non-standard error
codes after failure. This can be used in the tools to print specific
error messages. xenpaging recognizes these errno values and shows them
if the init function fails.

Document the return codes in the public header file.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>