KVM: x86: VMX: fix build without hyper-v

Commit ceef7d1 ("KVM: x86: VMX: hyper-v: Enlightened MSR-Bitmap
support") broke the build with Hyper-V disabled, because it accesses
ms_hyperv.nested_features without checking if that exists.

This is the quick-and-hacky build fix.

I suspect the proper fix is to replace the

    static_branch_unlikely(&enable_evmcs)

tests with an inline helper function that also checks that CONFIG_HYPERV
is enabled, since without that, enable_evmcs makes no sense.

But I want a working build environment first and foremost, and I'm upset
this slipped through in the first place.  My primary build tests missed
it because I tend to build with everything enabled, but it should have
been caught in the kvm tree.

Fixes: ceef7d1 ("KVM: x86: VMX: hyper-v: Enlightened MSR-Bitmap support")
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/kvm/vmx.c

@@ -4429,13 +4429,16 @@ static int alloc_loaded_vmcs(struct loaded_vmcs *loaded_vmcs)
 			goto out_vmcs;
 		memset(loaded_vmcs->msr_bitmap, 0xff, PAGE_SIZE);
 
+#if IS_ENABLED(CONFIG_HYPERV)
 		if (static_branch_unlikely(&enable_evmcs) &&
 		    (ms_hyperv.nested_features & HV_X64_NESTED_MSR_BITMAP)) {
 			struct hv_enlightened_vmcs *evmcs =
 				(struct hv_enlightened_vmcs *)loaded_vmcs->vmcs;
 
 			evmcs->hv_enlightenments_control.msr_bitmap = 1;
 		}
+#endif
+
 	}
 	return 0;