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/mem.rs

@@ -14,18 +14,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use super::Error as DeviceError;
-use super::{
-    ActivateError, ActivateResult, EpollHelper, EpollHelperError, EpollHelperHandler, VirtioCommon,
-    VirtioDevice, VirtioDeviceType, EPOLL_HELPER_EVENT_LAST, VIRTIO_F_VERSION_1,
-};
-use crate::seccomp_filters::Thread;
-use crate::thread_helper::spawn_virtio_thread;
-use crate::{GuestMemoryMmap, GuestRegionMmap};
-use crate::{VirtioInterrupt, VirtioInterruptType};
-use anyhow::anyhow;
-use seccompiler::SeccompAction;
-use serde::{Deserialize, Serialize};
 use std::collections::BTreeMap;
 use std::io;
 use std::mem::size_of;
@@ -34,6 +22,10 @@ use std::result;
 use std::sync::atomic::AtomicBool;
 use std::sync::mpsc;
 use std::sync::{Arc, Barrier, Mutex};
+
+use anyhow::anyhow;
+use seccompiler::SeccompAction;
+use serde::{Deserialize, Serialize};
 use thiserror::Error;
 use virtio_queue::{DescriptorChain, Queue, QueueT};
 use vm_device::dma_mapping::ExternalDmaMapping;
@@ -45,6 +37,16 @@ use vm_migration::protocol::MemoryRangeTable;
 use vm_migration::{Migratable, MigratableError, Pausable, Snapshot, Snapshottable, Transportable};
 use vmm_sys_util::eventfd::EventFd;
 
+use super::Error as DeviceError;
+use super::{
+    ActivateError, ActivateResult, EpollHelper, EpollHelperError, EpollHelperHandler, VirtioCommon,
+    VirtioDevice, VirtioDeviceType, EPOLL_HELPER_EVENT_LAST, VIRTIO_F_VERSION_1,
+};
+use crate::seccomp_filters::Thread;
+use crate::thread_helper::spawn_virtio_thread;
+use crate::{GuestMemoryMmap, GuestRegionMmap};
+use crate::{VirtioInterrupt, VirtioInterruptType};
+
 const QUEUE_SIZE: u16 = 128;
 const QUEUE_SIZES: &[u16] = &[QUEUE_SIZE];