Add entries for py3o images into the manifest file.

def __add_images_to_manifest(self):
        """Add entries for py3o images into the manifest file."""

        xpath_expr = "//manifest:manifest[1]"

        for content_tree in self.content_trees:

            # Find manifest:manifest tags.
            manifest_e = content_tree.xpath(
                xpath_expr,
                namespaces=self.namespaces
            )
            if not manifest_e:
                continue

            for identifier in self.images.keys():
                # Add a manifest:file-entry tag.
                lxml.etree.SubElement(
                    manifest_e[0],
                    '{%s}file-entry' % self.namespaces['manifest'],
                    attrib={
                        '{%s}full-path' % self.namespaces['manifest']: (
                            PY3O_IMAGE_PREFIX + identifier
                        ),
                        '{%s}media-type' % self.namespaces['manifest']: '',
                    }
                )