CHANGELOG: add hypervisor framework and Hyper-V support

Signed-off-by: Wei Liu <wl@xen.org>
Acked-by: Paul Durrant <paul@xen.org>

CHANGELOG.md

@@ -14,6 +14,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    the Xen hypercall interface or the viridian one.
  - Assorted pvshim performance and scalability improvements plus some bug
    fixes.
+ - Hypervisor framework to ease porting Xen to run on hypervisors.
+ - Initial support to run on Hyper-V.
 
 ## [4.13.0](https://xenbits.xen.org/gitweb/?p=xen.git;a=shortlog;h=RELEASE-4.13.0) - 2019-12-17