xen: implement a debug-interrupt handler

Xen supports the notion of a debug interrupt which can be triggered
from the console.  For now this is implemented to show pending events,
masks and each CPU's pending event set.

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>