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>

vmm/src/interrupt.rs

@@ -3,12 +3,13 @@
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 //
 
-use devices::interrupt_controller::InterruptController;
-use hypervisor::IrqRoutingEntry;
 use std::collections::HashMap;
 use std::io;
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Mutex};
+
+use devices::interrupt_controller::InterruptController;
+use hypervisor::IrqRoutingEntry;
 use vm_allocator::SystemAllocator;
 use vm_device::interrupt::{
     InterruptIndex, InterruptManager, InterruptSourceConfig, InterruptSourceGroup,