hw/intc/arm_gicv3_its: Fix the VM termination in vm_change_state_handler()

The commit cddafd8f ("hw/intc/arm_gicv3_its: Implement state save
/restore") breaks the backward compatibility with the older kernels
where vITS save/restore support is not available. The vmstate function
vm_change_state_handler() should not be registered if the running kernel
doesn't support ITS save/restore feature. Otherwise VM instance will be
killed whenever vmstate callback function is invoked.

Observed a virtual machine shutdown with QEMU-2.10+linux-4.11 when testing
the reboot command "virsh reboot <domain> --mode acpi" instead of reboot.

KVM Error: 'KVM_SET_DEVICE_ATTR failed: Group 4 attr 0x00000000000001'
Signed-off-by: NShanker Donthineni <shankerd@codeaurora.org>
Reviewed-by: NEric Auger <eric.auger@redhat.com>
Message-id: 1509712671-16299-1-git-send-email-shankerd@codeaurora.org
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/intc/arm_gicv3_its_kvm.c

@@ -111,13 +111,13 @@ static void kvm_arm_its_realize(DeviceState *dev, Error **errp)
             error_free(s->migration_blocker);
             return;
         }
+    } else {
+        qemu_add_vm_change_state_handler(vm_change_state_handler, s);
     }
 
     kvm_msi_use_devid = true;
     kvm_gsi_direct_mapping = false;
     kvm_msi_via_irqfd_allowed = kvm_irqfds_enabled();
-
-    qemu_add_vm_change_state_handler(vm_change_state_handler, s);
 }
 
 /**