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>

api_client/src/lib.rs

@@ -5,6 +5,7 @@
 
 use std::io::{Read, Write};
 use std::os::unix::io::RawFd;
+
 use thiserror::Error;
 use vmm_sys_util::sock_ctrl_msg::ScmSocket;