KVM: x86: Clear DR7.LE during task-switch

DR7.LE should be cleared during task-switch. This feature is poorly documented.
For reference, see:
http://pdos.csail.mit.edu/6.828/2005/readings/i386/s12_02.htm

SDM [17.2.4]:
  This feature is not supported in the P6 family processors, later IA-32
  processors, and Intel 64 processors.

AMD [2:13.1.1.4]:
  This bit is ignored by implementations of the AMD64 architecture.

Intel's formulation could mean that it isn't even zeroed, but current
hardware indeed does not behave like that.
Signed-off-by: NNadav Amit <namit@cs.technion.ac.il>
Reviewed-by: NRadim Krčmář <rkrcmar@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -5490,7 +5490,7 @@ static int handle_task_switch(struct kvm_vcpu *vcpu)
 	}
 
 	/* clear all local breakpoint enable flags */
-	vmcs_writel(GUEST_DR7, vmcs_readl(GUEST_DR7) & ~0x55);
+	vmcs_writel(GUEST_DR7, vmcs_readl(GUEST_DR7) & ~0x155);
 
 	/*
 	 * TODO: What about debug traps on tss switch?