CHANGELOG: mention xl/libxl SMBIOS support

Add an entry for the new xl/libxl SMBIOS support.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Henry Wang <Henry.Wang@arm.com>

CHANGELOG.md

@@ -13,6 +13,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
      livelocks, instead of crashing the entire server.
    - Bus-lock detection, used by Xen to mitigate (by rate-limiting) the system
      wide impact of a guest misusing atomic instructions.
+ - xl/libxl can customize SMBIOS strings for HVM guests.
 
 ## [4.17.0](https://xenbits.xen.org/gitweb/?p=xen.git;a=shortlog;h=RELEASE-4.17.0) - 2022-12-12