vfio/spapr: Introduce spapr backend and target interface

vfio/spapr: Introduce spapr backend and target interface

Introduce an empty spapr backend which will hold spapr specific
content, currently only prereg_listener and hostwin_list.

Also introduce two spapr specific callbacks add/del_window into
VFIOIOMMUOps. Instantiate a spapr ops with a helper setup_spapr_ops
and assign it to bcontainer->ops.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>