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
ae600e24bf