vfio: ccw: register vfio_ccw to the mediated device framework

To make vfio support subchannel devices, we need to leverage the
mediated device framework to create a mediated device for the
subchannel device.

This registers the subchannel device to the mediated device
framework during probe to enable mediated device creation.

Reviewed-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Message-Id: <20170317031743.40128-7-bjsdjshi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>