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>

virtio-devices/src/vhost_user/vu_common_ctrl.rs

@@ -1,12 +1,6 @@
 // Copyright 2019 Intel Corporation. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 
-use super::{Error, Result};
-use crate::vhost_user::Inflight;
-use crate::{
-    get_host_address_range, GuestMemoryMmap, GuestRegionMmap, MmapRegion, VirtioInterrupt,
-    VirtioInterruptType,
-};
 use std::ffi;
 use std::fs::File;
 use std::os::unix::io::{AsRawFd, FromRawFd, RawFd};
@@ -15,6 +9,7 @@ use std::sync::atomic::Ordering;
 use std::sync::Arc;
 use std::thread::sleep;
 use std::time::{Duration, Instant};
+
 use vhost::vhost_kern::vhost_binding::{VHOST_F_LOG_ALL, VHOST_VRING_F_LOG};
 use vhost::vhost_user::message::{
     VhostUserHeaderFlag, VhostUserInflight, VhostUserProtocolFeatures, VhostUserVirtioFeatures,
@@ -30,6 +25,13 @@ use vm_memory::{
 use vm_migration::protocol::MemoryRangeTable;
 use vmm_sys_util::eventfd::EventFd;
 
+use super::{Error, Result};
+use crate::vhost_user::Inflight;
+use crate::{
+    get_host_address_range, GuestMemoryMmap, GuestRegionMmap, MmapRegion, VirtioInterrupt,
+    VirtioInterruptType,
+};
+
 // Size of a dirty page for vhost-user.
 const VHOST_LOG_PAGE: u64 = 0x1000;