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>

vmm/Cargo.toml

@@ -12,6 +12,7 @@ fwdebug = ["devices/fwdebug"]
 kvm = ["hypervisor/kvm"]
 mshv = ["hypervisor/mshv"]
 io_uring = ["virtio-devices/io_uring"]
+tdx = ["arch/tdx"]
 
 [dependencies]
 acpi_tables = { path = "../acpi_tables", optional = true }