Rename ChangeLog to CHANGES

This is the OCaml/Mirage standard

Signed-off-by: David Scott <dave.scott@citrix.com>
Release 0.9.21

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #57 from djs55/release.0.9.21

Release.0.9.21
xenlight: only build xenlight if xen 4.4 or xen 4.5

The stubs need at least xen 4.4; previous versions appear to be
incompatible.

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #58 from djs55/xl-for-4.4-only

xenlight: only build xenlight if xen 4.4 or xen 4.5