hvmloader: relicense hvmloader xenbus implementation under more

hvmloader: relicense hvmloader xenbus implementation under more
liberal terms.

This code is a great example of a simple xenbus implementation and we
would like to reuse it in projects with non-GPLv2 license
(specifically in this case SeaBIOS which is GPLv3).

I picked the license from extras/mini-os/COPYING (A two clause BSD
style license) since mini-os exists for much the same purpose.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Keir Fraser <keir@xen.org>