css: mss/mcss-e vs. migration

Our main channel_subsys structure is not a device (yet), but we need
to setup mss/mcss-e again if the guest had enabled it before. Use
a hack that should catch most configurations (assuming that the guest
will have enabled at least one device in higher subchannel sets or
channel subsystems if it enabled the functionality.)

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>