tdx: Add "tdx" feature with an empty module inside arch to implement

Add the skeleton of the "tdx" feature with a module ready inside the
arch crate to store implementation details.

TEST=cargo build --features="tdx"

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

arch/src/x86_64/mod.rs

@@ -25,6 +25,8 @@ use vm_memory::{
 };
 mod smbios;
 use std::arch::x86_64;
+#[cfg(feature = "tdx")]
+pub mod tdx;
 
 #[derive(Debug, Copy, Clone)]
 pub enum BootProtocol {

arch/src/x86_64/tdx/mod.rs

@@ -0,0 +1,4 @@
+// Copyright © 2021 Intel Corporation
+//
+// SPDX-License-Identifier: Apache-2.0
+