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/ioapic.rs

@@ -10,7 +10,6 @@
 // See https://pdos.csail.mit.edu/6.828/2016/readings/ia32/ioapic.pdf for a specification.
 
 use super::interrupt_controller::{Error, InterruptController};
-use crate::BusDevice;
 use anyhow::anyhow;
 use byteorder::{ByteOrder, LittleEndian};
 use std::result;
@@ -19,6 +18,7 @@ use vm_device::interrupt::{
     InterruptIndex, InterruptManager, InterruptSourceConfig, InterruptSourceGroup,
     MsiIrqGroupConfig, MsiIrqSourceConfig,
 };
+use vm_device::BusDevice;
 use vm_memory::GuestAddress;
 use vm_migration::{
     Migratable, MigratableError, Pausable, Snapshot, SnapshotDataSection, Snapshottable,