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>

devices/src/pvmemcontrol.rs

@@ -3,17 +3,18 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 
-use num_enum::TryFromPrimitive;
-use pci::{
-    BarReprogrammingParams, PciBarConfiguration, PciBarPrefetchable, PciBarRegionType,
-    PciClassCode, PciConfiguration, PciDevice, PciDeviceError, PciHeaderType, PciSubclass,
-};
 use std::{
     collections::HashMap,
     ffi::CString,
     io, result,
     sync::{Arc, Barrier, Mutex, RwLock},
 };
+
+use num_enum::TryFromPrimitive;
+use pci::{
+    BarReprogrammingParams, PciBarConfiguration, PciBarPrefetchable, PciBarRegionType,
+    PciClassCode, PciConfiguration, PciDevice, PciDeviceError, PciHeaderType, PciSubclass,
+};
 use thiserror::Error;
 use vm_allocator::{page_size::get_page_size, AddressAllocator, SystemAllocator};
 use vm_device::{BusDeviceSync, Resource};