[Future] Make it easier for users to enter datastore paths

[zjs]

We ask users to provide paths to directories on selected datastores when configuring volume and image stores. We do not validate those paths, and will create them if they do not exist. This makes it easy for users to make mistakes, and hard for them to simply select an existing path.

We should explore providing a tree selector, auto-completing paths as they are entered, or something else.