CHANGELOG: Mention libxl blktap/tapback support

Add entry for backendtype=tap support in libxl.  blktap needs some
changes to work with libxl, which haven't been merged.  They are
available from this PR: https://github.com/xapi-project/blktap/pull/394

Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

CHANGELOG.md

@@ -26,6 +26,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
      for IPIs and Physical addressing mode for external interrupts.
  - Add a new 9pfs backend running as a daemon in dom0. First user is
    Xenstore-stubdom now being able to support full Xenstore trace capability.
+ - libxl support for backendtype=tap with tapback.
 
 ### Removed
 - caml-stubdom.  It hasn't built since 2014, was pinned to Ocaml 4.02, and has