Add xl init scripts

Add xl init scripts

this patch adds an init script for xl, so that it can be used without
xend installed on the system.  In cases in which both toolstacks are
present, only one init script will be run, dependening on the value of
xen_toolstack in /etc/sysconfig/xen_toolstack.  Please note that a
correct value of xen_toolstack (either xl or xend), is required now.
The network script and the vif script to be used with xl can be
specified in /etc/sysconfig/xenlightdaemons.  Xl sets the vif script
to xl-vif-script by default, that reads /etc/sysconfig/xenlightdaemons
and calls the vif script set there.  A port to a debian system should
be straightforward, just replacing /etc/sysconfig with /etc/default
everywhere.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>