virttest.env_process: Fix bug of postprocess_image.

When VM image cannot be found, image.info() will return None.
Then postprocess_image will raise following exception:
```
    raise virt_vm.VMError("Failures occurred while postprocess:%s" % err)
        VMError: Failures occurred while postprocess:
        Postprocess: 'NoneType' object has no attribute 'splitlines'
```
and test will be failed(None of testcase's business).
So check it before splitlines().

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>