statvfs is not available on windows

statvfs was introduced in the following commit:
https://github.com/Tinche/aiofiles/commit/ec8cc1e27b2ecd6346b24753a1b66fd60f6b7acc

statvfs is not available on Windows as far as I can gather.

```
AttributeError: module 'os' has no attribute 'statvfs'
```

A simple fix may be something like:

```python
if hasattr(os, 'statvfs'):
    statvfs = wrap(os.statvfs)
```
