KVM: vmx, pmu: accept 0 for host-initiated write to MSR_IA32_DS_AREA

Whenever an MSR is part of KVM_GET_MSR_INDEX_LIST, as is the case
for MSR_IA32_DS_AREA, it has to be always settable with KVM_SET_MSR.
Accept a zero value for these MSRs to obey the contract.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/pmu_intel.c

@@ -443,6 +443,8 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		}
 		break;
 	case MSR_IA32_DS_AREA:
+		if (msr_info->host_initiated && data && !guest_cpuid_has(vcpu, X86_FEATURE_DS))
+			return 1;
 		if (is_noncanonical_address(data, vcpu))
 			return 1;
 		pmu->ds_area = data;