xen: Used SPDX identifier in public headers

xen: Used SPDX identifier in public headers

The text of the licence has been check to be the same as the one at
https://spdx.org/licenses/MIT.html, except we don't have "(including
the next paragraph)".

Mecanical change done with a script.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>