misc: Adapt consistent import style formatting

Historically the Cloud Hypervisor coding style has been to ensure that
all imports are ordered and placed in a single group. Unfortunately
cargo fmt has no support for ensuring that all imports are in a single
group so if whitespace lines were added as part of the import statements
then they would only be odered correctly in the group.

By adopting "group_imports="StdExternalCrate" we can enforce a style
where imports are placed in at most three groups for std, external
crates and the crate itself. Choosing a style enforceable by the tooling
reduces the reviewer burden.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

pci/src/device.rs

@@ -4,15 +4,17 @@
 //
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
-use crate::configuration::{self, PciBarRegionType};
-use crate::PciBarConfiguration;
 use std::any::Any;
 use std::fmt::{self, Display};
 use std::sync::{Arc, Barrier, Mutex};
 use std::{io, result};
+
 use vm_allocator::{AddressAllocator, SystemAllocator};
 use vm_device::Resource;
 
+use crate::configuration::{self, PciBarRegionType};
+use crate::PciBarConfiguration;
+
 #[derive(Debug)]
 pub enum Error {
     /// Setup of the device capabilities failed.