'Updates a consistency group.
:param context: the context of the caller.
:param group: the dictionary of the consistency group to be updated.
:param add_volumes: a list of volume dictionaries to be added.
:param remove_volumes: a list of volume dictionaries to be removed.
:returns: model_update, add_volumes_update, remove_volumes_update
model_update is a dictionary that the driver wants the manager
to update upon a successful return. If None is returned, the manager
will set the status to \'available\'.
add_volumes_update and remove_volumes_update are lists of dictionaries
that the driver wants the manager to update upon a successful return.
Note that each entry requires a {\'id\': xxx} so that the correct
volume entry can be updated. If None is returned, the volume will
remain its original status. Also note that you cannot directly
assign add_volumes to add_volumes_update as add_volumes is a list of
cinder.db.sqlalchemy.models.Volume objects and cannot be used for
db update directly. Same with remove_volumes.
If the driver throws an exception, the status of the group as well as
those of the volumes to be added/removed will be set to \'error\'.'

def update_consistencygroup(self, context, group, add_volumes=None, remove_volumes=None):

    raise NotImplementedError()
