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/cpu.rs

@@ -8,6 +8,9 @@
 //
 //
 
+use thiserror::Error;
+use vm_memory::GuestAddress;
+
 #[cfg(target_arch = "aarch64")]
 use crate::aarch64::{RegList, VcpuInit};
 #[cfg(target_arch = "x86_64")]
@@ -17,8 +20,6 @@ use crate::kvm::{TdxExitDetails, TdxExitStatus};
 use crate::CpuState;
 use crate::MpState;
 use crate::StandardRegisters;
-use thiserror::Error;
-use vm_memory::GuestAddress;
 
 #[cfg(target_arch = "x86_64")]
 #[derive(Copy, Clone, Default)]