xen/arm: compile and run xenbus

bind_evtchn_to_irqhandler can legitimately return 0 (irq 0): it is not
an error.

If Linux is running as an HVM domain and is running as Dom0, use
xenstored_local_init to initialize the xenstore page and event channel.

Changes in v4:
- do not xs_reset_watches on dom0.

Changes in v2:
- refactor xenbus_init.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
[v5: Fixed case switch indentations]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/xenbus/xenbus_comms.c

@@ -224,7 +224,7 @@ int xb_init_comms(void)
 		int err;
 		err = bind_evtchn_to_irqhandler(xen_store_evtchn, wake_waiting,
 						0, "xenbus", &xb_waitq);
-		if (err <= 0) {
+		if (err < 0) {
 			printk(KERN_ERR "XENBUS request irq failed %i\n", err);
 			return err;
 		}