This commit adds few Xen public headers, which are needed for further
development of Xen features in Zephyr. Headers were taken from
include/public directory of Xen RELEASE-4.15.0 branch, filtered and
cleaned up to leave only parts, which are now required.
Directory structure were saved (public, public/io, public/hvm), so
public headers content can be supplemented if nessesary.
Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>
