[HVM] Hide contents of IA32_MCG_STATUS MSR from the guest.

It can't clear the MCE, and it probably can't understand it anyway.
Signed-off-by: Tim Deegan <Tim.Deegan@xensource.com>

xen/arch/x86/hvm/svm/svm.c

@@ -2154,6 +2154,11 @@ static void svm_do_msr_access(
             svm_inject_exception(v, TRAP_gp_fault, 1, 0);
             break;
 
+        case MSR_IA32_MCG_STATUS:
+            /* No point in letting the guest see real MCEs */
+            msr_content = 0;
+            break;
+
         default:
             if ( rdmsr_hypervisor_regs(ecx, &eax, &edx) ||
                  rdmsr_safe(ecx, eax, edx) == 0 )

xen/arch/x86/hvm/vmx/vmx.c

@@ -2622,6 +2622,10 @@ static int vmx_do_msr_read(struct cpu_user_regs *regs)
         break;
     case MSR_IA32_VMX_BASIC...MSR_IA32_VMX_PROCBASED_CTLS2:
         goto gp_fault;
+    case MSR_IA32_MCG_STATUS:
+        /* No point in letting the guest see real MCEs */
+        msr_content = 0;
+        break;
     default:
         switch ( long_mode_do_msr_read(regs) )
         {