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>

block/src/raw_sync.rs

@@ -2,15 +2,17 @@
 //
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
+use std::collections::VecDeque;
+use std::fs::File;
+use std::io::{Seek, SeekFrom};
+use std::os::unix::io::{AsRawFd, RawFd};
+
+use vmm_sys_util::eventfd::EventFd;
+
 use crate::async_io::{
     AsyncIo, AsyncIoError, AsyncIoResult, DiskFile, DiskFileError, DiskFileResult,
 };
 use crate::DiskTopology;
-use std::collections::VecDeque;
-use std::fs::File;
-use std::io::{Seek, SeekFrom};
-use std::os::unix::io::{AsRawFd, RawFd};
-use vmm_sys_util::eventfd::EventFd;
 
 pub struct RawFileDiskSync {
     file: File,