xen: Disable ARINC653 scheduler by default for non-DEBUG builds

xen: Disable ARINC653 scheduler by default for non-DEBUG builds

The ARINC653 scheduler is targeted at a very specific niche; typical
users cannot benefit from using it.  Disable it by default for
non-DEBUG builds.  (Enable it for DEBUG builds so that we catch any
build breakages sooner rather than later.)

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>