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>

rate_limiter/src/lib.rs

@@ -51,6 +51,7 @@ use std::os::unix::io::{AsRawFd, RawFd};
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Mutex;
 use std::time::{Duration, Instant};
+
 use vmm_sys_util::timerfd::TimerFd;
 
 /// Module for group rate limiting.
@@ -519,10 +520,11 @@ impl Default for RateLimiter {
 
 #[cfg(test)]
 pub(crate) mod tests {
-    use super::*;
     use std::fmt;
     use std::thread;
 
+    use super::*;
+
     impl TokenBucket {
         // Resets the token bucket: budget set to max capacity and last-updated set to now.
         fn reset(&mut self) {