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/lib.rs

@@ -48,7 +48,8 @@ mod cpu;
 /// Device related module
 mod device;
 
-pub use crate::hypervisor::{Hypervisor, HypervisorError};
+use std::sync::Arc;
+
 use concat_idents::concat_idents;
 #[cfg(target_arch = "x86_64")]
 pub use cpu::CpuVendor;
@@ -56,12 +57,13 @@ pub use cpu::{HypervisorCpuError, Vcpu, VmExit};
 pub use device::HypervisorDeviceError;
 #[cfg(all(feature = "kvm", target_arch = "aarch64"))]
 pub use kvm::{aarch64, GicState};
-use std::sync::Arc;
 pub use vm::{
     DataMatch, HypervisorVmError, InterruptSourceConfig, LegacyIrqSourceConfig, MsiIrqSourceConfig,
     Vm, VmOps,
 };
 
+pub use crate::hypervisor::{Hypervisor, HypervisorError};
+
 #[derive(Debug, Copy, Clone)]
 pub enum HypervisorType {
     #[cfg(feature = "kvm")]