libxl: add PV console if not explicitly specified
    
    Xen PV domains always have a PV console, so add one to the domain
    config via post-parse callback if not explicitly specified in
    the XML.  The legacy Xen driver behaves similarly, causing a
    regression when switching to the new Xen toolstack.  I.e.
    
      virsh console pv-domain
    
    will no longer work after upgrading a xm/xend stack to xl/libxl.