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>

net_util/src/queue_pair.rs

@@ -2,19 +2,21 @@
 //
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
-use super::{register_listener, unregister_listener, vnet_hdr_len, Tap};
-use rate_limiter::{RateLimiter, TokenType};
 use std::io;
 use std::num::Wrapping;
 use std::os::unix::io::{AsRawFd, RawFd};
 use std::sync::atomic::{AtomicU64, Ordering};
 use std::sync::Arc;
+
+use rate_limiter::{RateLimiter, TokenType};
 use thiserror::Error;
 use virtio_queue::{Queue, QueueOwnedT, QueueT};
 use vm_memory::bitmap::Bitmap;
 use vm_memory::{Bytes, GuestMemory};
 use vm_virtio::{AccessPlatform, Translatable};
 
+use super::{register_listener, unregister_listener, vnet_hdr_len, Tap};
+
 #[derive(Clone)]
 pub struct TxVirtio {
     pub counter_bytes: Wrapping<u64>,