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>

tests/integration.rs

@@ -10,7 +10,6 @@
 
 extern crate test_infra;
 
-use net_util::MacAddr;
 use std::collections::HashMap;
 use std::fs;
 use std::io;
@@ -26,6 +25,8 @@ use std::sync::mpsc;
 use std::sync::mpsc::Receiver;
 use std::sync::Mutex;
 use std::thread;
+
+use net_util::MacAddr;
 use test_infra::*;
 use vmm_sys_util::{tempdir::TempDir, tempfile::TempFile};
 use wait_timeout::ChildExt;
@@ -7955,9 +7956,10 @@ mod common_sequential {
 }
 
 mod windows {
-    use crate::*;
     use once_cell::sync::Lazy;
 
+    use crate::*;
+
     static NEXT_DISK_ID: Lazy<Mutex<u8>> = Lazy::new(|| Mutex::new(1));
 
     struct WindowsGuest {