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/msix.rs

@@ -3,19 +3,21 @@
 // SPDX-License-Identifier: Apache-2.0 OR BSD-3-Clause
 //
 
-use crate::{PciCapability, PciCapabilityId};
-use byteorder::{ByteOrder, LittleEndian};
-use serde::Deserialize;
-use serde::Serialize;
 use std::io;
 use std::result;
 use std::sync::Arc;
+
+use byteorder::{ByteOrder, LittleEndian};
+use serde::Deserialize;
+use serde::Serialize;
 use vm_device::interrupt::{
     InterruptIndex, InterruptSourceConfig, InterruptSourceGroup, MsiIrqSourceConfig,
 };
 use vm_memory::ByteValued;
 use vm_migration::{MigratableError, Pausable, Snapshot, Snapshottable};
 
+use crate::{PciCapability, PciCapabilityId};
+
 const MAX_MSIX_VECTORS_PER_DEVICE: u16 = 2048;
 const MSIX_TABLE_ENTRIES_MODULO: u64 = 16;
 const MSIX_PBA_ENTRIES_MODULO: u64 = 8;