changelog: add note about blkif protocol fixes

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

CHANGELOG.md

@@ -7,6 +7,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ## [4.20.0 UNRELEASED](https://xenbits.xenproject.org/gitweb/?p=xen.git;a=shortlog;h=staging) - TBD
 
 ### Changed
+ - Fixed blkif protocol specification for sector sizes different than 512b.
  - On x86:
    - Prefer ACPI reboot over UEFI ResetSystem() run time service call.