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/qcow/mod.rs

@@ -9,20 +9,15 @@ mod raw_file;
 mod refcount;
 mod vec_cache;
 
-use crate::qcow::{
-    qcow_raw_file::QcowRawFile,
-    refcount::RefCount,
-    vec_cache::{CacheMap, Cacheable, VecCache},
-};
-use crate::BlockBackend;
-use byteorder::{BigEndian, ReadBytesExt, WriteBytesExt};
-use libc::{EINVAL, ENOSPC, ENOTSUP};
-use remain::sorted;
 use std::cmp::{max, min};
 use std::fs::OpenOptions;
 use std::io::{self, Read, Seek, SeekFrom, Write};
 use std::mem::size_of;
 use std::str;
+
+use byteorder::{BigEndian, ReadBytesExt, WriteBytesExt};
+use libc::{EINVAL, ENOSPC, ENOTSUP};
+use remain::sorted;
 use thiserror::Error;
 use vmm_sys_util::{
     file_traits::FileSetLen, file_traits::FileSync, seek_hole::SeekHole, write_zeroes::PunchHole,
@@ -30,6 +25,12 @@ use vmm_sys_util::{
 };
 
 pub use crate::qcow::raw_file::RawFile;
+use crate::qcow::{
+    qcow_raw_file::QcowRawFile,
+    refcount::RefCount,
+    vec_cache::{CacheMap, Cacheable, VecCache},
+};
+use crate::BlockBackend;
 
 /// Nesting depth limit for disk formats that can open other disk files.
 const MAX_NESTING_DEPTH: u32 = 10;
@@ -1837,13 +1838,15 @@ pub fn detect_image_type(file: &mut RawFile) -> Result<ImageType> {
 
 #[cfg(test)]
 mod tests {
-    use super::*;
     use std::fs::File;
     use std::path::Path;
+
     use vmm_sys_util::tempdir::TempDir;
     use vmm_sys_util::tempfile::TempFile;
     use vmm_sys_util::write_zeroes::WriteZeroes;
 
+    use super::*;
+
     fn valid_header_v3() -> Vec<u8> {
         vec![
             0x51u8, 0x46, 0x49, 0xfb, // magic

block/src/qcow/qcow_raw_file.rs

@@ -4,12 +4,14 @@
 //
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
-use super::RawFile;
-use byteorder::{BigEndian, ReadBytesExt, WriteBytesExt};
 use std::io::{self, BufWriter, Seek, SeekFrom, Write};
 use std::mem::size_of;
+
+use byteorder::{BigEndian, ReadBytesExt, WriteBytesExt};
 use vmm_sys_util::write_zeroes::WriteZeroes;
 
+use super::RawFile;
+
 /// A qcow file. Allows reading/writing clusters and appending clusters.
 #[derive(Debug)]
 pub struct QcowRawFile {

block/src/qcow/raw_file.rs

@@ -8,15 +8,17 @@
 //
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
-use crate::BlockBackend;
-use libc::c_void;
 use std::alloc::{alloc_zeroed, dealloc, Layout};
 use std::fs::{File, Metadata};
 use std::io::{self, Read, Seek, SeekFrom, Write};
 use std::os::unix::io::{AsRawFd, RawFd};
 use std::slice;
+
+use libc::c_void;
 use vmm_sys_util::{seek_hole::SeekHole, write_zeroes::PunchHole, write_zeroes::WriteZeroesAt};
 
+use crate::BlockBackend;
+
 #[derive(Debug)]
 pub struct RawFile {
     file: File,