xen: no need for domU to worry about MCE/MCA

Mask MCE/MCA out of cpu caps.  Its harmless to leave them there, but
it does prevent the kernel from starting an unnecessary thread.

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