VMX: relax incoming BNDCFGS check

Accepting zero here even when !cpu_has_mpx makes the restore side
symmetric to the save logic (which avoids saving the value if zero),
i.e. makes either side independent of the logic on the other side.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -824,7 +824,7 @@ static int vmx_load_msr(struct vcpu *v,
         case MSR_IA32_BNDCFGS:
             if ( cpu_has_mpx )
                 __vmwrite(GUEST_BNDCFGS, ctxt->msr[i].val);
-            else
+            else if ( ctxt->msr[i].val )
                 err = -ENXIO;
             break;
         case MSR_IA32_XSS: