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>

block/src/fixed_vhd.rs

@@ -2,12 +2,13 @@
 //
 // SPDX-License-Identifier: Apache-2.0
 
-use crate::vhd::VhdFooter;
-use crate::BlockBackend;
 use std::fs::File;
 use std::io::{Read, Seek, SeekFrom, Write};
 use std::os::unix::io::{AsRawFd, RawFd};
 
+use crate::vhd::VhdFooter;
+use crate::BlockBackend;
+
 #[derive(Debug)]
 pub struct FixedVhd {
     file: File,