KVM: x86: announce KVM_CAP_HYPERV_ENLIGHTENED_VMCS support only when it is available

It was discovered that after commit 65efa61d ("selftests: kvm: provide
common function to enable eVMCS") hyperv_cpuid selftest is failing on AMD.
The reason is that the commit changed _vcpu_ioctl() to vcpu_ioctl() in the
test and this one can't fail.

Instead of fixing the test is seems to make more sense to not announce
KVM_CAP_HYPERV_ENLIGHTENED_VMCS support if it is definitely missing
(on svm and in case kvm_intel.nested=0).
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -3205,7 +3205,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_HYPERV_EVENTFD:
 	case KVM_CAP_HYPERV_TLBFLUSH:
 	case KVM_CAP_HYPERV_SEND_IPI:
-	case KVM_CAP_HYPERV_ENLIGHTENED_VMCS:
 	case KVM_CAP_HYPERV_CPUID:
 	case KVM_CAP_PCI_SEGMENT:
 	case KVM_CAP_DEBUGREGS:
@@ -3283,7 +3282,10 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 			kvm_x86_ops->get_nested_state(NULL, NULL, 0) : 0;
 		break;
 	case KVM_CAP_HYPERV_DIRECT_TLBFLUSH:
-		r = kvm_x86_ops->enable_direct_tlbflush ? 1 : 0;
+		r = kvm_x86_ops->enable_direct_tlbflush != NULL;
+		break;
+	case KVM_CAP_HYPERV_ENLIGHTENED_VMCS:
+		r = kvm_x86_ops->nested_enable_evmcs != NULL;
 		break;
 	default:
 		break;