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/device.rs

@@ -6,11 +6,6 @@
 //
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
-use crate::{
-    ActivateError, ActivateResult, Error, GuestMemoryMmap, GuestRegionMmap,
-    VIRTIO_F_RING_INDIRECT_DESC,
-};
-use libc::EFD_NONBLOCK;
 use std::collections::HashMap;
 use std::io::Write;
 use std::num::Wrapping;
@@ -19,6 +14,8 @@ use std::sync::{
     Arc, Barrier,
 };
 use std::thread;
+
+use libc::EFD_NONBLOCK;
 use virtio_queue::Queue;
 use vm_memory::{GuestAddress, GuestMemoryAtomic, GuestUsize};
 use vm_migration::{MigratableError, Pausable};
@@ -26,6 +23,11 @@ use vm_virtio::AccessPlatform;
 use vm_virtio::VirtioDeviceType;
 use vmm_sys_util::eventfd::EventFd;
 
+use crate::{
+    ActivateError, ActivateResult, Error, GuestMemoryMmap, GuestRegionMmap,
+    VIRTIO_F_RING_INDIRECT_DESC,
+};
+
 pub enum VirtioInterruptType {
     Config,
     Queue(u16),