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/rng.rs

@@ -4,6 +4,23 @@
 //
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
+use std::fs::File;
+use std::io;
+use std::os::unix::io::AsRawFd;
+use std::result;
+use std::sync::atomic::AtomicBool;
+use std::sync::{Arc, Barrier};
+
+use anyhow::anyhow;
+use seccompiler::SeccompAction;
+use serde::{Deserialize, Serialize};
+use thiserror::Error;
+use virtio_queue::{Queue, QueueT};
+use vm_memory::{GuestAddressSpace, GuestMemory, GuestMemoryAtomic};
+use vm_migration::{Migratable, MigratableError, Pausable, Snapshot, Snapshottable, Transportable};
+use vm_virtio::{AccessPlatform, Translatable};
+use vmm_sys_util::eventfd::EventFd;
+
 use super::Error as DeviceError;
 use super::{
     ActivateError, ActivateResult, EpollHelper, EpollHelperError, EpollHelperHandler, VirtioCommon,
@@ -14,21 +31,6 @@ use crate::seccomp_filters::Thread;
 use crate::thread_helper::spawn_virtio_thread;
 use crate::GuestMemoryMmap;
 use crate::{VirtioInterrupt, VirtioInterruptType};
-use anyhow::anyhow;
-use seccompiler::SeccompAction;
-use serde::{Deserialize, Serialize};
-use std::fs::File;
-use std::io;
-use std::os::unix::io::AsRawFd;
-use std::result;
-use std::sync::atomic::AtomicBool;
-use std::sync::{Arc, Barrier};
-use thiserror::Error;
-use virtio_queue::{Queue, QueueT};
-use vm_memory::{GuestAddressSpace, GuestMemory, GuestMemoryAtomic};
-use vm_migration::{Migratable, MigratableError, Pausable, Snapshot, Snapshottable, Transportable};
-use vm_virtio::{AccessPlatform, Translatable};
-use vmm_sys_util::eventfd::EventFd;
 
 const QUEUE_SIZE: u16 = 256;
 const QUEUE_SIZES: &[u16] = &[QUEUE_SIZE];