func: VIOsPAPRDevice *vty_lookup(sPAPRMachineState *spapr, target_ulong reg)
  {
      VIOsPAPRDevice *sdev;
      sdev = spapr_vio_find_by_reg(spapr->vio_bus, reg);
      if (!sdev && reg == 0) {
          /* Hack for kernel early debug, which always specifies reg==0.
           * We search all VIO devices, and grab the vty with the lowest
           * reg.  This attempts to mimic existing PowerVM behaviour
           * (early debug does work there, despite having no vty with
           * reg==0. */
          return spapr_vty_get_default(spapr->vio_bus);
      return sdev;