hw/s390x/virtio-ccw: Consolidate calls to virtio_ccw_unrealize()

hw/s390x/virtio-ccw: Consolidate calls to virtio_ccw_unrealize()

Currently, every virtio-ccw device explicitely sets its unrealize
function to virtio_ccw_unrealize() in its class_init function.
We can simplify this by using a common unrealize function, just like
it is already done for the realize functions.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1532521224-27235-2-git-send-email-thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>