virttest.utils_misc: Fix bug of zero disk size for get_image_info.

When disk size is zero, get_image_info will raise ValueError because
normalize_data_size return a null string. But zero of disk size
should be allowed.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #1648 from yumingfei/get_image_info_bug

virttest.utils_misc: Fix bug of zero disk size for get_image_info.