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>

devices/src/tpm.rs

@@ -3,13 +3,14 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 
+use std::cmp;
+use std::sync::{Arc, Barrier};
+
 use anyhow::anyhow;
 #[cfg(target_arch = "aarch64")]
 use arch::aarch64::layout::{TPM_SIZE, TPM_START};
 #[cfg(target_arch = "x86_64")]
 use arch::x86_64::layout::{TPM_SIZE, TPM_START};
-use std::cmp;
-use std::sync::{Arc, Barrier};
 use thiserror::Error;
 use tpm::emulator::{BackendCmd, Emulator};
 use tpm::TPM_CRB_BUFFER_MAX;