s390/cio: introduce io_subchannel_type

The sysfs attributes implemented by the vfio_ccw driver are also implemented by
the io_subchannel driver. Move these into a device_type which is set by the
css bus.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Reviewed-by: Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>