Lots of unnecessary API calls when the list of available drives is large

[ali-tny]

At the moment, we always iterate through all the available drives calling the expensive _get_drive_root operation. The list drive object is actually a generator, but we turn it to a dictionary anyway which executes the entire generator.

there's caches to the method calls in question, but it doesn't really help because the standard API for using tentaclio is via tio.open, which creates a new client each time anyway, meaning the cache is useless

We should:

• only iterate through drives until we find the right one
• only call the expensive _get_drive_root method on that specific drive

needs a little bit of refactoring to do this cleanly