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.

@Guannan-Ren , This is work for https://github.com/autotest/tp-libvirt/pull/37
Please help me to check it.

Before:
```
09:52:05 DEBUG| Running 'qemu-img info /MYDISK/BIG/autotest/client/tests/gren_virt/tmp/blockresize_test'
09:52:05 DEBUG| image: /MYDISK/BIG/autotest/client/tests/gren_virt/tmp/blockresize_test
file format: raw
virtual size: 1.0M (1048576 bytes)
disk size: 0
...
09:52:24 ERROR|     image_info_dict['dsize'] = int(float(normalize_data_size(dsize, "B")))
ValueError: could not convert string to float:
```

Now:
{'dsize': 0, 'vsize': 1048576, 'format': 'raw'}

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>