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>

virtio-devices/src/thread_helper.rs

@@ -3,17 +3,19 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 
+use std::{
+    panic::AssertUnwindSafe,
+    thread::{self, JoinHandle},
+};
+
+use seccompiler::{apply_filter, SeccompAction};
+use vmm_sys_util::eventfd::EventFd;
+
 use crate::{
     epoll_helper::EpollHelperError,
     seccomp_filters::{get_seccomp_filter, Thread},
     ActivateError,
 };
-use seccompiler::{apply_filter, SeccompAction};
-use std::{
-    panic::AssertUnwindSafe,
-    thread::{self, JoinHandle},
-};
-use vmm_sys_util::eventfd::EventFd;
 
 pub(crate) fn spawn_virtio_thread<F>(
     name: &str,