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/igvm/loader.rs

@@ -2,7 +2,6 @@
 //
 // Copyright © 2023, Microsoft Corporation
 //
-use crate::igvm::{BootPageAcceptance, StartupMemoryType, HV_PAGE_SIZE};
 use range_map_vec::{Entry, RangeMap};
 use thiserror::Error;
 use vm_memory::bitmap::AtomicBitmap;
@@ -11,6 +10,8 @@ use vm_memory::{
     GuestMemoryRegion,
 };
 
+use crate::igvm::{BootPageAcceptance, StartupMemoryType, HV_PAGE_SIZE};
+
 /// Structure to hold the guest memory info/layout to check
 /// the if the memory is accepted within the layout.
 /// Adds up the total bytes written to the guest memory