x86: separate generic cpu disabling code from APIC writes in cpu_disable

It allows paravirt implementations of cpu_disable to share the
cpu_disable_common code, without having to take on board APIC
writes, which may not be appropriate.

Signed-off-by: Alex Nixon <alex.nixon@citrix.com>
Acked-by: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>