CHANGELOG: Mention PVH boot mode

Signed-off-by: Colin Percival <cperciva@freebsd.org>

Author: cperciva

CHANGELOG.md

@@ -4,6 +4,9 @@
 
 ### Added
 
+- Added support for PVH boot mode.  This is used when an x86 kernel provides
+  the appropriate ELF Note to indicate that PVH boot mode is supported.
+
 ### Changed
 
 - Updated deserialization of `bitmap` for custom CPU templates to allow usage