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 a6ee3b5)
Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>

Author: kalyazin

CHANGELOG.md

@@ -41,6 +41,10 @@ and this project adheres to
   information about page size to the payload Firecracker sends to the UFFD
   handler. Each memory region object now contains a `page_size_kib` field. See
   also the [hugepages documentation](docs/hugepages.md).
+- [#4501](https://github.com/firecracker-microvm/firecracker/pull/4501): 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.
 
 ### Fixed