xen/events: add struct evtchn_ops for the low-level port operations

evtchn_ops contains the low-level operations that access the shared
data structures.  This allows alternate ABIs to be supported.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/events/events_base.c

@@ -61,6 +61,8 @@
 
 #include "events_internal.h"
 
+const struct evtchn_ops *evtchn_ops;
+
 /*
  * This lock protects updates to the following mapping and reference-count
  * arrays. The lock does not need to be acquired to read the mapping tables.
@@ -1523,6 +1525,8 @@ void __init xen_init_IRQ(void)
 {
 	int i;
 
+	xen_evtchn_2l_init();
+
 	evtchn_to_irq = kcalloc(NR_EVENT_CHANNELS, sizeof(*evtchn_to_irq),
 				    GFP_KERNEL);
 	BUG_ON(!evtchn_to_irq);