KVM: x86: nVMX: Allow nested_enable_evmcs to be NULL

...so that it can conditionally set by the VMX code, i.e. iff @nested is
true.  This will in turn allow it to be moved out of vmx.c and into a
nested-specified file.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -3827,6 +3827,8 @@ static int kvm_vcpu_ioctl_enable_cap(struct kvm_vcpu *vcpu,
 		return kvm_hv_activate_synic(vcpu, cap->cap ==
 					     KVM_CAP_HYPERV_SYNIC2);
 	case KVM_CAP_HYPERV_ENLIGHTENED_VMCS:
+		if (!kvm_x86_ops->nested_enable_evmcs)
+			return -ENOTTY;
 		r = kvm_x86_ops->nested_enable_evmcs(vcpu, &vmcs_version);
 		if (!r) {
 			user_ptr = (void __user *)(uintptr_t)cap->args[0];