Snce the xen driver was deleted we need to ensure that the old
libvirt-daemon-driver-xen sub-RPM gets removed on upgrade. We
achieve this my making libvirt-daemon-driver-libxl obsolete it.

We don't add a Provides: too, because libvirt-daemon-driver-libxl
is not a functionally identical replacement, since we don't want
to satisfy deps for 3rd party apps that have a Requires on the
libvirt-daemon-driver-xen RPM.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>