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/lib.rs

@@ -0,0 +1,60 @@
+// 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.
+
+//! Emulates virtual and hardware devices.
+extern crate byteorder;
+extern crate epoll;
+extern crate libc;
+
+extern crate vm_memory;
+extern crate vmm_sys_util;
+
+use std::fs::File;
+use std::io;
+
+mod bus;
+pub mod legacy;
+
+pub use self::bus::{Bus, BusDevice, Error as BusError};
+
+pub type DeviceEventT = u16;
+
+/// The payload is used to handle events where the internal state of the VirtIO device
+/// needs to be changed.
+pub enum EpollHandlerPayload {
+    /// DrivePayload(disk_image)
+    DrivePayload(File),
+    /// Events that do not need a payload.
+    Empty,
+}
+
+type Result<T> = std::result::Result<T, Error>;
+
+pub trait EpollHandler: Send {
+    fn handle_event(
+        &mut self,
+        device_event: DeviceEventT,
+        event_flags: u32,
+        payload: EpollHandlerPayload,
+    ) -> Result<()>;
+}
+
+#[derive(Debug)]
+pub enum Error {
+    FailedReadingQueue {
+        event_type: &'static str,
+        underlying: io::Error,
+    },
+    FailedReadTap,
+    FailedSignalingUsedQueue(io::Error),
+    PayloadExpected,
+    UnknownEvent {
+        device: &'static str,
+        event: DeviceEventT,
+    },
+    IoError(io::Error),
+}