devices, vm-device: Move BusDevice and Bus into vm-device

This removes the dependency of the pci crate on the devices crate which
now only contains the device implementations themselves.

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

devices/src/legacy/serial.rs

@@ -5,12 +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.
 
-use crate::BusDevice;
 use anyhow::anyhow;
 use std::collections::VecDeque;
 use std::sync::Arc;
 use std::{io, result};
 use vm_device::interrupt::InterruptSourceGroup;
+use vm_device::BusDevice;
 use vm_migration::{
     Migratable, MigratableError, Pausable, Snapshot, SnapshotDataSection, Snapshottable,
     Transportable,