remove the xen dracut module

remove the xen dracut module

The Xen module is unnecessary and it has been for a while.
Most Xen systems will not be using the module, even now, because
xen-detect is not installed by default on most Xen systems, and
dracut uses xen-detect to decide whether to include the module.

It also has some problems:

1) it does not try loading xen_platform_pci;

2) it loads modules unnecessarily; modules.alias is where all Xen support
should reside.  Assuming xenbus_probe_frontend and xen_platform_pci
are loaded so that Xen devices are probed, other modules are picked up
automatically thanks to aliases such as

    alias xen:vbd xen_blkfront

3) Even not-so-recent kernels (say 2.6.32) require the xen_platform_pci
and xenbus_probe_frontend modules even for non-paravirtualized guests.
60xen/module-setup.sh picks the module only for PV guests.

So, just require xenbus_probe_frontend to be builtin, and also
xen_platform_pci for fully-virtualized guests, and remove the module.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>