xen/x86: set VIRQ_TIMER priority to maximum

Commit bee980d9e (xen/events: Handle VIRQ_TIMER before any other hardirq
in event loop) effectively made the VIRQ_TIMER the highest priority event
when using the 2-level ABI.

Set the VIRQ_TIMER priority to the highest so this behaviour is retained
when using the FIFO-based ABI.

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>

arch/x86/xen/time.c

@@ -446,6 +446,7 @@ void xen_setup_timer(int cpu)
 				      IRQF_PERCPU|IRQF_NOBALANCING|IRQF_TIMER|
 				      IRQF_FORCE_RESUME,
 				      name, NULL);
+	(void)xen_set_irq_priority(irq, XEN_IRQ_PRIORITY_MAX);
 
 	memcpy(evt, xen_clockevent, sizeof(*evt));