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/configuration.rs

@@ -4,15 +4,17 @@
 //
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
-use crate::device::BarReprogrammingParams;
-use crate::{MsixConfig, PciInterruptPin};
-use byteorder::{ByteOrder, LittleEndian};
-use serde::{Deserialize, Serialize};
 use std::fmt::{self, Display};
 use std::sync::{Arc, Mutex};
+
+use byteorder::{ByteOrder, LittleEndian};
+use serde::{Deserialize, Serialize};
 use vm_device::PciBarType;
 use vm_migration::{MigratableError, Pausable, Snapshot, Snapshottable};
 
+use crate::device::BarReprogrammingParams;
+use crate::{MsixConfig, PciInterruptPin};
+
 // The number of 32bit registers in the config space, 4096 bytes.
 const NUM_CONFIGURATION_REGISTERS: usize = 1024;