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>

vm-migration/src/protocol.rs

@@ -3,11 +3,13 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 
-use crate::MigratableError;
-use serde::{Deserialize, Serialize};
 use std::io::{Read, Write};
+
+use serde::{Deserialize, Serialize};
 use vm_memory::ByteValued;
 
+use crate::MigratableError;
+
 // Migration protocol
 // 1: Source establishes communication with destination (file socket or TCP connection.)
 // (The establishment is out of scope.)