This commit adds the xen_4_8 package to be used instead of
xen (currently at 4.5.5):
* Add packages xen_4_8, xen_4_8-slim and xen_4_8-light
* Add packages qemu_xen_4_8 and qemu_xen_4_8-light to be used
with xen_4_8-slim and xen_4_8-light respectively.
* Add systemd to buildInputs of xen (it is required by oxenstored)
* Adapt xen service to work with the new version of xen
* Use xen-init-dom0 to initlilise dom0 in xen-store
* Currently, the virtualisation.xen.stored option is ignored
if xen 4.8 is used
Also:
* provides a bunch of build options
* documents build options config in longDescription
* provides a bunch of predefined packages and documents them some more
* sources' hashes stay the same
