add support for rate limit in qemu-img convert.

Signed-off-by: Zhengui <lizhengui@huawei.com>
Message-Id: <1603205264-17424-3-git-send-email-lizhengui@huawei.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>