libxl: add PV console if not explicitly specified
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.

datto/libvirt,VenkatDatta/libvirt,crobinso/libvirt,rlaager/libvirt,andreabolognani/libvirt,zippy2/libvirt,eskultety/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,elmarco/libvirt,elmarco/libvirt,crobinso/libvirt,andreabolognani/libvirt,datto/libvirt,jardasgit/libvirt,nertpinx/libvirt,olafhering/libvirt,datto/libvirt,libvirt/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,agx/libvirt,taget/libvirt,elmarco/libvirt,nertpinx/libvirt,libvirt/libvirt,libvirt/libvirt,fabianfreyer/libvirt,rlaager/libvirt,shugaoye/libvirt,shugaoye/libvirt,agx/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,olafhering/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,shugaoye/libvirt,taget/libvirt,rlaager/libvirt,taget/libvirt,eskultety/libvirt,nertpinx/libvirt,olafhering/libvirt,agx/libvirt,shugaoye/libvirt,jfehlig/libvirt,elmarco/libvirt,fabianfreyer/libvirt,eskultety/libvirt,zippy2/libvirt,zippy2/libvirt,jfehlig/libvirt,crobinso/libvirt,elmarco/libvirt,agx/libvirt,andreabolognani/libvirt,nertpinx/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,jardasgit/libvirt,rlaager/libvirt,rlaager/libvirt,eskultety/libvirt,zippy2/libvirt,taget/libvirt,agx/libvirt,jfehlig/libvirt,crobinso/libvirt,taget/libvirt,datto/libvirt,fabianfreyer/libvirt,datto/libvirt,eskultety/libvirt,olafhering/libvirt,nertpinx/libvirt,shugaoye/libvirt,andreabolognani/libvirt,libvirt/libvirt