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/ioapic.rs

@@ -9,11 +9,11 @@
 // Implementation of an intel 82093AA Input/Output Advanced Programmable Interrupt Controller
 // See https://pdos.csail.mit.edu/6.828/2016/readings/ia32/ioapic.pdf for a specification.
 
-use super::interrupt_controller::{Error, InterruptController};
-use byteorder::{ByteOrder, LittleEndian};
-use serde::{Deserialize, Serialize};
 use std::result;
 use std::sync::{Arc, Barrier};
+
+use byteorder::{ByteOrder, LittleEndian};
+use serde::{Deserialize, Serialize};
 use vm_device::interrupt::{
     InterruptIndex, InterruptManager, InterruptSourceConfig, InterruptSourceGroup,
     MsiIrqGroupConfig, MsiIrqSourceConfig,
@@ -23,6 +23,8 @@ use vm_memory::GuestAddress;
 use vm_migration::{Migratable, MigratableError, Pausable, Snapshot, Snapshottable, Transportable};
 use vmm_sys_util::eventfd::EventFd;
 
+use super::interrupt_controller::{Error, InterruptController};
+
 type Result<T> = result::Result<T, Error>;
 
 // I/O REDIRECTION TABLE REGISTER