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>

vm-allocator/src/address.rs

@@ -9,6 +9,7 @@
 
 use std::collections::btree_map::BTreeMap;
 use std::result;
+
 use vm_memory::{Address, GuestAddress, GuestUsize};
 
 #[derive(Debug)]

vm-allocator/src/system.rs

@@ -13,7 +13,6 @@ use crate::address::AddressAllocator;
 use crate::gsi::GsiAllocator;
 #[cfg(target_arch = "x86_64")]
 use crate::gsi::GsiApic;
-
 use crate::page_size::get_page_size;
 
 /// Manages allocating system resources such as address space and interrupt numbers.