utils_misc: Fix get_image_info fail to get format

For qemu-img-1.5.3, We get following message when running:
`# qemu-img info /var/lib/virt_test/images/jeos-19-64.qcow2`
```
image: /var/lib/virt_test/images/jeos-19-64.qcow2
file format: qcow2
virtual size: 10G (10737418240 bytes)
disk size: 761M
cluster_size: 65536
Format specific information:
    compat: 0.10
```

The `Format specific information:` line which containing a
`Format` will flush the saved file format. Now we use a more
specific line to check file format.

Signed-off-by: Hao Liu <hliu@redhat.com>