x86: Skip check apic_id_limit for Xen

Xen vIOMMU device model will be in Xen hypervisor. Skip vIOMMU
check for Xen here when vcpu number is more than 255.
Signed-off-by: NLan Tianyu <tianyu.lan@intel.com>
Message-Id: <1502842933-8323-1-git-send-email-tianyu.lan@intel.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

hw/i386/pc.c

@@ -1228,7 +1228,7 @@ void pc_machine_done(Notifier *notifier, void *data)
         fw_cfg_modify_i16(pcms->fw_cfg, FW_CFG_NB_CPUS, pcms->boot_cpus);
     }
 
-    if (pcms->apic_id_limit > 255) {
+    if (pcms->apic_id_limit > 255 && !xen_enabled()) {
         IntelIOMMUState *iommu = INTEL_IOMMU_DEVICE(x86_iommu_get_default());
 
         if (!iommu || !iommu->x86_iommu.intr_supported ||