s390x/css: add missing css state conditionally

Although we have recently vmstatified the migration of some css
infrastructure,  for some css entities there is still state to be
migrated left, because the focus was keeping migration stream
compatibility (that is basically everything as-is).

Let us add vmstate helpers and extend existing vmstate descriptions so
that we have everything we need. Let us guard the added state via
css_migration_enabled, so we keep the compatible behavior if css
migration is disabled.

Let's also annotate the bits which do not need to be migrated for better
readability.

Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20170711145441.33925-4-pasic@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
