xen: short-cut for recursive event handling

If an event comes in while events are currently being processed, then
just increment the counter and have the outer event loop reprocess the
pending events.  This prevents unbounded recursion on heavy event
loads (of course massive event storms will cause infinite loops).

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>