s390x: add property adapter_routes_max_batch

To make virtio-ccw supports more that  64 virtqueues we will have to
increase ADAPTER_ROUTES_MAX_GSI which is currently limiting the number if
possible adapter routes. Of course increasing the number of supported
routes can break backwards migration.

Let us introduce a compatibility property adapter_routes_max_batch so
client code can use the some old limit if in compatibility mode and
retain the migration compatibility.

Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
