vfio-ccw: Register a chp_event callback for vfio-ccw

Register the chp_event callback to receive channel path related
events for the subchannels managed by vfio-ccw.

Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20200505122745.53208-3-farman@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>