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/vhdx_sync.rs

@@ -2,14 +2,16 @@
 //
 // SPDX-License-Identifier: Apache-2.0
 
-use crate::async_io::{AsyncIo, AsyncIoResult, DiskFile, DiskFileError, DiskFileResult};
-use crate::vhdx::{Result as VhdxResult, Vhdx};
-use crate::AsyncAdaptor;
 use std::collections::VecDeque;
 use std::fs::File;
 use std::sync::{Arc, Mutex, MutexGuard};
+
 use vmm_sys_util::eventfd::EventFd;
 
+use crate::async_io::{AsyncIo, AsyncIoResult, DiskFile, DiskFileError, DiskFileResult};
+use crate::vhdx::{Result as VhdxResult, Vhdx};
+use crate::AsyncAdaptor;
+
 pub struct VhdxDiskSync {
     vhdx_file: Arc<Mutex<Vhdx>>,
 }