s390x/css: fix css migration compat handling

s390x/css: fix css migration compat handling

Commit e996583eb3 ("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>