'List the backing images currently in use.'

def _list_backing_images(self):

    inuse_images = []
    for ent in os.listdir(CONF.instances_path):
        if (ent in self.instance_names):
            LOG.debug('%s    is    a    valid    instance    name', ent)
            disk_path = os.path.join(CONF.instances_path, ent, 'disk')
            if os.path.exists(disk_path):
                LOG.debug('%s    has    a    disk    file', ent)
                try:
                    backing_file = libvirt_utils.get_disk_backing_file(disk_path)
                except processutils.ProcessExecutionError:
                    if (not os.path.exists(disk_path)):
                        LOG.debug('Failed    to    get    disk    backing    file:    %s', disk_path)
                        continue
                    else:
                        raise
                LOG.debug('Instance    %(instance)s    is    backed    by    %(backing)s', {'instance': ent, 'backing': backing_file})
                if backing_file:
                    backing_path = os.path.join(CONF.instances_path, CONF.image_cache_subdirectory_name, backing_file)
                    if (backing_path not in inuse_images):
                        inuse_images.append(backing_path)
                    if (backing_path in self.unexplained_images):
                        LOG.warning('Instance    %(instance)s    is    using    a    backing    file    %(backing)s    which    does    not    appear    in    the    image    service', {'instance': ent, 'backing': backing_file})
                        self.unexplained_images.remove(backing_path)
    return inuse_images
