Fix serial interrupt's destination

Lowest Priority can't use with invalid cpu_mask, and the default value
of CPU_MASK_ALL may cover CPU which wasn't online.

From: "Yang, Sheng" <sheng.yang@intel.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>

xen/arch/x86/smpboot.c

@@ -1015,7 +1015,7 @@ void __init smp_intr_init(void)
         irq_vector[irq] = FIRST_HIPRIORITY_VECTOR + seridx + 1;
         per_cpu(vector_irq, cpu)[FIRST_HIPRIORITY_VECTOR + seridx + 1] = irq;
         irq_cfg[irq].vector = FIRST_HIPRIORITY_VECTOR + seridx + 1;
-        irq_cfg[irq].cpu_mask = (cpumask_t)CPU_MASK_ALL;
+        irq_cfg[irq].cpu_mask = cpu_online_map;
     }
 
     /* IPI for cleanuping vectors after irq move */