doc(changelog): mention usage of anon private for non-vhost-user vms

Adds a changelog entry that VMs that do not use
vhost-user-blk devices are backed by anon private memory mapping.

(cherry picked from commit 36e419166041b6ae61086045d2604d99457296c1)
Signed-off-by: Nikita Kalyazin <[email protected]>

Author: kalyazin

CHANGELOG.md

@@ -5,6 +5,15 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## \[Unreleased\]
+
+### Changed
+
+- [#4502](https://github.com/firecracker-microvm/firecracker/pull/4502): Only
+  use memfd to back guest memory if a vhost-user-blk device is configured,
+  otherwise use anonymous private memory. This is because serving page faults of
+  shared memory used by memfd is slower and may impact workloads.
+
 ## [1.6.0]
 
 ### Added