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/vhd.rs

@@ -2,10 +2,11 @@
 //
 // SPDX-License-Identifier: Apache-2.0
 
-use crate::{read_aligned_block_size, DiskTopology};
 use std::fs::File;
 use std::io::{Seek, SeekFrom};
 
+use crate::{read_aligned_block_size, DiskTopology};
+
 #[derive(Clone, Copy)]
 pub struct VhdFooter {
     cookie: u64,
@@ -117,11 +118,13 @@ pub fn is_fixed_vhd(f: &mut File) -> std::io::Result<bool> {
 
 #[cfg(test)]
 mod tests {
-    use super::{is_fixed_vhd, VhdFooter};
     use std::fs::File;
     use std::io::{Seek, SeekFrom, Write};
+
     use vmm_sys_util::tempfile::TempFile;
 
+    use super::{is_fixed_vhd, VhdFooter};
+
     fn valid_fixed_vhd_footer() -> Vec<u8> {
         vec![
             0x63, 0x6f, 0x6e, 0x65, 0x63, 0x74, 0x69, 0x78, // cookie