x86, xen: short-circuit tests for dom0

When testing for a dom0/initial/privileged domain, make sure the
predicate evaluates to a compile-time 0 if CONFIG_XEN_DOM0 isn't
enabled.  This will make most of the dom0 code evaporate without
much more effort.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>