Introduce the BdrvDmgUncompressFunc type defintion. To emphasis
dmg_uncompress_bz2 and dmg_uncompress_lzfse are pointer to functions,
declare them using this new typedef.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20230320152610.32052-1-philmd@linaro.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>