The recipes are introduced as the non-default version by adding an
expressed PREFERRED_VERSION for 4.14 in:
    conf/distro/include/meta-virt-default-versions.inc

to keep the Xen 4.14 recipes as default for the Hardknott release.

Signed-off-by: Christopher Clark <christopher.w.clark@gmail.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>