devices, vmm: Add legacy CMOS device

Based off of crosvm revision b5237bbcf074eb30cf368a138c0835081e747d71
add a CMOS device. This environments that can't use KVM clock to get the
current time (e.g. Windows and EFI.)

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

devices/src/legacy/mod.rs

@@ -5,8 +5,12 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE-BSD-3-Clause file.
 
+#[cfg(feature = "cmos")]
+mod cmos;
 mod i8042;
 mod serial;
 
+#[cfg(feature = "cmos")]
+pub use self::cmos::Cmos;
 pub use self::i8042::I8042Device;
 pub use self::serial::Serial;