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>

hypervisor/src/kvm/aarch64/gic/dist_regs.rs

@@ -1,12 +1,13 @@
 // Copyright 2020 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 
+use kvm_ioctls::DeviceFd;
+
 use crate::arch::aarch64::gic::{Error, Result};
 use crate::device::HypervisorDeviceError;
 use crate::kvm::kvm_bindings::{
     kvm_device_attr, KVM_DEV_ARM_VGIC_GRP_DIST_REGS, KVM_DEV_ARM_VGIC_GRP_NR_IRQS,
 };
-use kvm_ioctls::DeviceFd;
 
 /*
  Distributor registers as detailed at page 456 from

hypervisor/src/kvm/aarch64/gic/icc_regs.rs

@@ -2,6 +2,8 @@
 // Copyright 2020 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 
+use kvm_ioctls::DeviceFd;
+
 use crate::arch::aarch64::gic::{Error, Result};
 use crate::device::HypervisorDeviceError;
 use crate::kvm::kvm_bindings::{
@@ -10,7 +12,6 @@ use crate::kvm::kvm_bindings::{
     KVM_REG_ARM64_SYSREG_OP0_MASK, KVM_REG_ARM64_SYSREG_OP0_SHIFT, KVM_REG_ARM64_SYSREG_OP1_MASK,
     KVM_REG_ARM64_SYSREG_OP1_SHIFT, KVM_REG_ARM64_SYSREG_OP2_MASK, KVM_REG_ARM64_SYSREG_OP2_SHIFT,
 };
-use kvm_ioctls::DeviceFd;
 
 const KVM_DEV_ARM_VGIC_V3_MPIDR_SHIFT: u32 = 32;
 const KVM_DEV_ARM_VGIC_V3_MPIDR_MASK: u64 = 0xffffffff << KVM_DEV_ARM_VGIC_V3_MPIDR_SHIFT as u64;

hypervisor/src/kvm/aarch64/gic/mod.rs

@@ -6,16 +6,18 @@ mod dist_regs;
 mod icc_regs;
 mod redist_regs;
 
-use crate::arch::aarch64::gic::{Error, Result, Vgic, VgicConfig};
-use crate::device::HypervisorDeviceError;
-use crate::kvm::KvmVm;
-use crate::{CpuState, Vm};
+use std::any::Any;
+
 use dist_regs::{get_dist_regs, read_ctlr, set_dist_regs, write_ctlr};
 use icc_regs::{get_icc_regs, set_icc_regs};
 use kvm_ioctls::DeviceFd;
 use redist_regs::{construct_gicr_typers, get_redist_regs, set_redist_regs};
 use serde::{Deserialize, Serialize};
-use std::any::Any;
+
+use crate::arch::aarch64::gic::{Error, Result, Vgic, VgicConfig};
+use crate::device::HypervisorDeviceError;
+use crate::kvm::KvmVm;
+use crate::{CpuState, Vm};
 
 const GITS_CTLR: u32 = 0x0000;
 const GITS_IIDR: u32 = 0x0004;

hypervisor/src/kvm/aarch64/gic/redist_regs.rs

@@ -2,6 +2,8 @@
 // Copyright 2020 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 
+use kvm_ioctls::DeviceFd;
+
 use crate::arch::aarch64::gic::{Error, Result};
 use crate::device::HypervisorDeviceError;
 use crate::kvm::kvm_bindings::{
@@ -12,7 +14,6 @@ use crate::kvm::kvm_bindings::{
 use crate::kvm::Register;
 use crate::kvm::VcpuKvmState;
 use crate::CpuState;
-use kvm_ioctls::DeviceFd;
 
 // Relevant redistributor registers that we want to save/restore.
 const GICR_CTLR: u32 = 0x0000;

hypervisor/src/kvm/aarch64/mod.rs

@@ -10,7 +10,6 @@
 
 pub mod gic;
 
-use crate::kvm::{KvmError, KvmResult};
 use kvm_bindings::{
     kvm_mp_state, kvm_one_reg, kvm_regs, KVM_REG_ARM_COPROC_MASK, KVM_REG_ARM_CORE,
     KVM_REG_SIZE_MASK, KVM_REG_SIZE_U32, KVM_REG_SIZE_U64,
@@ -19,6 +18,8 @@ pub use kvm_bindings::{kvm_one_reg as Register, kvm_vcpu_init as VcpuInit, RegLi
 use serde::{Deserialize, Serialize};
 pub use {kvm_ioctls::Cap, kvm_ioctls::Kvm};
 
+use crate::kvm::{KvmError, KvmResult};
+
 // This macro gets the offset of a structure (i.e `str`) member (i.e `field`) without having
 // an instance of that structure.
 #[macro_export]