KVM: x86: Clear DR6[0:3] on #DB during handle_dr

DR6[0:3] (previous breakpoint indications) are cleared when #DB is injected
during handle_exception, just as real hardware does.  Similarily, handle_dr
should clear DR6[0:3].

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index f9d56c1..ad5e080 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -5125,6 +5125,7 @@
 			vcpu->run->exit_reason = KVM_EXIT_DEBUG;
 			return 0;
 		} else {
+			vcpu->arch.dr6 &= ~15;
 			vcpu->arch.dr6 |= DR6_BD | DR6_RTM;
 			kvm_queue_exception(vcpu, DB_VECTOR);
 			return 1;