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>

Cargo.toml

@@ -56,6 +56,7 @@ fwdebug = ["vmm/fwdebug"]
 kvm = ["vmm/kvm"]
 mshv = ["vmm/mshv"]
 io_uring = ["vmm/io_uring"]
+tdx = ["vmm/tdx"]
 
 # Integration tests require a special environment to run in
 integration_tests = []