Add convenient collection wrapper for easier getting selected volume.
Storage pool implementation may still provide only volume listing
function (pool.list_volumes), or, additionally, optimized
pool.get_volume.
This means it is both possible to iterate over volumes:
```python
for volume in pool.volumes:
    ...
```
And get a single volume:
```python
volume = pool.volumes[vid]
```
QubesOS/qubes-issues#2256
		
	
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| .. | ||
| __init__.py | ||
| file.py | ||
| kernels.py | ||
| lvm.py | ||
| README.md | ||
WNI File storage
Before v3.1 there existed a draft wni storage. You can find it in the git history
(it was in /core/storage directory, now gone)