vfio-ccw: add capabilities chain

Allow to extend the regions used by vfio-ccw. The first user will be
handling of halt and clear subchannel.

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