s390x/css: fix css migration compat handling

Commit e996583 ("s390x/css: activate ChannelSubSys migration",
2017-07-11) was supposed to enable css migration for virtio-ccw
machines starting 2.10, but it ended up effectively enabling it
only for 2.10 as the registration of the appropriate VMStateDescription
happens in ccw_machine_2_10_instance_options which does not get
called for machines more recent than 2_10.

Let us move the corresponding chunk of code (which conditionally enables
the migration based on the value of the corresponding class property) to
ccw_init, which is called for each virtio-ccw machine instance.

Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Reported-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20171004110109.16525-1-pasic@linux.vnet.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

hw/s390x/s390-virtio-ccw.c

@@ -291,6 +291,9 @@ static void ccw_init(MachineState *machine)
         ret = css_create_css_image(VIRTUAL_CSSID, true);
     }
     assert(ret == 0);
+    if (css_migration_enabled()) {
+        css_register_vmstate();
+    }
 
     /* Create VirtIO network adapters */
     s390_create_virtio_net(BUS(css_bus), "virtio-net-ccw");
@@ -715,9 +718,6 @@ DEFINE_CCW_MACHINE(2_11, "2.11", true);
 static void ccw_machine_2_10_instance_options(MachineState *machine)
 {
     ccw_machine_2_11_instance_options(machine);
-    if (css_migration_enabled()) {
-        css_register_vmstate();
-    }
 }
 
 static void ccw_machine_2_10_class_options(MachineClass *mc)