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>

vmm/src/pci_segment.rs

@@ -9,17 +9,19 @@
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 //
 
-use crate::device_manager::{AddressManager, DeviceManagerError, DeviceManagerResult};
+use std::sync::{Arc, Mutex};
+
 use acpi_tables::{aml, Aml};
 use arch::layout;
 use pci::{DeviceRelocation, PciBdf, PciBus, PciConfigMmio, PciRoot};
 #[cfg(target_arch = "x86_64")]
 use pci::{PciConfigIo, PCI_CONFIG_IO_PORT, PCI_CONFIG_IO_PORT_SIZE};
-use std::sync::{Arc, Mutex};
 use uuid::Uuid;
 use vm_allocator::AddressAllocator;
 use vm_device::BusDeviceSync;
 
+use crate::device_manager::{AddressManager, DeviceManagerError, DeviceManagerResult};
+
 pub(crate) struct PciSegment {
     pub(crate) id: u16,
     pub(crate) pci_bus: Arc<Mutex<PciBus>>,