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>

arch/src/aarch64/fdt.rs

@@ -6,17 +6,22 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the THIRD-PARTY file.
 
-use crate::{NumaNodes, PciSpaceInfo};
-use byteorder::{BigEndian, ByteOrder};
-use hypervisor::arch::aarch64::gic::Vgic;
 use std::cmp;
 use std::collections::HashMap;
 use std::ffi::CStr;
 use std::fmt::Debug;
+use std::fs;
+use std::path::Path;
 use std::result;
 use std::str;
 use std::sync::{Arc, Mutex};
 
+use byteorder::{BigEndian, ByteOrder};
+use hypervisor::arch::aarch64::gic::Vgic;
+use thiserror::Error;
+use vm_fdt::{FdtWriter, FdtWriterResult};
+use vm_memory::{Address, Bytes, GuestMemory, GuestMemoryError, GuestMemoryRegion};
+
 use super::super::DeviceType;
 use super::super::GuestMemoryMmap;
 use super::super::InitramfsConfig;
@@ -24,11 +29,7 @@ use super::layout::{
     IRQ_BASE, MEM_32BIT_DEVICES_SIZE, MEM_32BIT_DEVICES_START, MEM_PCI_IO_SIZE, MEM_PCI_IO_START,
     PCI_HIGH_BASE, PCI_MMIO_CONFIG_SIZE_PER_SEGMENT,
 };
-use std::fs;
-use std::path::Path;
-use thiserror::Error;
-use vm_fdt::{FdtWriter, FdtWriterResult};
-use vm_memory::{Address, Bytes, GuestMemory, GuestMemoryError, GuestMemoryRegion};
+use crate::{NumaNodes, PciSpaceInfo};
 
 // This is a value for uniquely identifying the FDT node declaring the interrupt controller.
 const GIC_PHANDLE: u32 = 1;

arch/src/aarch64/mod.rs

@@ -11,16 +11,18 @@ pub mod regs;
 /// Module for loading UEFI binary.
 pub mod uefi;
 
-pub use self::fdt::DeviceInfoForFdt;
-use crate::{DeviceType, GuestMemoryMmap, NumaNodes, PciSpaceInfo, RegionType};
-use hypervisor::arch::aarch64::gic::Vgic;
-use log::{log_enabled, Level};
 use std::collections::HashMap;
 use std::fmt::Debug;
 use std::sync::{Arc, Mutex};
+
+use hypervisor::arch::aarch64::gic::Vgic;
+use log::{log_enabled, Level};
 use thiserror::Error;
 use vm_memory::{Address, GuestAddress, GuestMemory, GuestMemoryAtomic};
 
+pub use self::fdt::DeviceInfoForFdt;
+use crate::{DeviceType, GuestMemoryMmap, NumaNodes, PciSpaceInfo, RegionType};
+
 pub const _NSIG: i32 = 65;
 
 /// Errors thrown while configuring aarch64 system.

arch/src/aarch64/uefi.rs

@@ -5,6 +5,7 @@
 use std::io::{Read, Seek, SeekFrom};
 use std::os::fd::AsFd;
 use std::result;
+
 use thiserror::Error;
 use vm_memory::{GuestAddress, GuestMemory};