vfio: ccw: Register mediated device once all structures are initialized

Let's register the mediated device when all the data structures
which could be used are initialized.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Message-Id: <1540487720-11634-3-git-send-email-pmorel@linux.ibm.com>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>