x2apic: xen64 paravirt basic apic ops

Define the Xen specific basic apic ops, in additon to paravirt apic ops,
with some misc warning fixes.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: akpm@linux-foundation.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>