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>

virtio-devices/src/vsock/csm/connection.rs

@@ -672,13 +672,15 @@ where
 
 #[cfg(test)]
 mod tests {
+    use std::io::{Error as IoError, Result as IoResult};
+
+    use libc::EFD_NONBLOCK;
+    use virtio_queue::QueueOwnedT;
+    use vmm_sys_util::eventfd::EventFd;
+
     use super::super::super::tests::TestContext;
     use super::super::defs as csm_defs;
     use super::*;
-    use libc::EFD_NONBLOCK;
-    use std::io::{Error as IoError, Result as IoResult};
-    use virtio_queue::QueueOwnedT;
-    use vmm_sys_util::eventfd::EventFd;
 
     const LOCAL_CID: u64 = 2;
     const PEER_CID: u64 = 3;

virtio-devices/src/vsock/csm/txbuf.rs

@@ -144,11 +144,12 @@ impl TxBuf {
 
 #[cfg(test)]
 mod tests {
-    use super::*;
     use std::io::Error as IoError;
     use std::io::ErrorKind;
     use std::io::Result as IoResult;
 
+    use super::*;
+
     struct TestSink {
         data: Vec<u8>,
         err: Option<IoError>,