KVM: PIT: stop vpit before freeing irq_routing

Fix:
general protection fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
......
Call Trace:
 [<ffffffffa0159bd1>] ? kvm_set_irq+0xdd/0x24b [kvm]
 [<ffffffff8106ea8b>] ? trace_hardirqs_off_caller+0x1f/0x10e
 [<ffffffff813ad17f>] ? sub_preempt_count+0xe/0xb6
 [<ffffffff8106d273>] ? put_lock_stats+0xe/0x27
...
RIP  [<ffffffffa0159c72>] kvm_set_irq+0x17e/0x24b [kvm]

This bug is triggered when guest is shutdown, is because we freed
irq_routing before pit thread stopped

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
index 70db4d4..0fd6378 100644
--- a/arch/x86/kvm/i8254.c
+++ b/arch/x86/kvm/i8254.c
@@ -752,6 +752,9 @@
 	struct hrtimer *timer;
 
 	if (kvm->arch.vpit) {
+		kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS, &kvm->arch.vpit->dev);
+		kvm_io_bus_unregister_dev(kvm, KVM_PIO_BUS,
+					      &kvm->arch.vpit->speaker_dev);
 		kvm_unregister_irq_mask_notifier(kvm, 0,
 					       &kvm->arch.vpit->mask_notifier);
 		kvm_unregister_irq_ack_notifier(kvm,