cloud-hypervisor: Add devices crate

Based on the Firecracker devices crate from commit 9cdb5b2.

It is a trimmed down version compared to the Firecracker one, to remove
a bunch of pulled dependencies (logger, metrics, rate limiter, etc...).

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

devices/src/legacy/mod.rs

@@ -0,0 +1,10 @@
+// Copyright 2018 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+//
+// Portions Copyright 2017 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the THIRD-PARTY file.
+
+mod serial;
+
+pub use self::serial::Serial;