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/lib.rs

@@ -10,15 +10,17 @@
 #[macro_use]
 extern crate log;
 
-#[cfg(target_arch = "x86_64")]
-use crate::x86_64::SgxEpcSection;
-use serde::{Deserialize, Serialize};
 use std::collections::BTreeMap;
 use std::fmt;
 use std::result;
 use std::sync::Arc;
+
+use serde::{Deserialize, Serialize};
 use thiserror::Error;
 
+#[cfg(target_arch = "x86_64")]
+use crate::x86_64::SgxEpcSection;
+
 type GuestMemoryMmap = vm_memory::GuestMemoryMmap<vm_memory::bitmap::AtomicBitmap>;
 type GuestRegionMmap = vm_memory::GuestRegionMmap<vm_memory::bitmap::AtomicBitmap>;