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>

devices/src/gic.rs

@@ -4,14 +4,15 @@
 
 use super::interrupt_controller::{Error, InterruptController};
 extern crate arch;
+use std::result;
+use std::sync::{Arc, Mutex};
+
 use anyhow::anyhow;
 use arch::layout;
 use hypervisor::{
     arch::aarch64::gic::{Vgic, VgicConfig},
     CpuState, GicState,
 };
-use std::result;
-use std::sync::{Arc, Mutex};
 use vm_device::interrupt::{
     InterruptIndex, InterruptManager, InterruptSourceConfig, InterruptSourceGroup,
     LegacyIrqSourceConfig, MsiIrqGroupConfig,