VirtualStorageProvider: encourage use of `new_overlay` or `new_memory`

[arrayka]

Make VirtualStorageProvider::new private.
Also add VirtualStorageProvider::new_physical<P: AsRefstd::path::Path>(path: P).
Expose only new_overlay(), new_memory(), new_physical().

Examples like these should be replaced with VirtualStorageProvider::new_memory():
let fs = OverlayFS::new(&[MemoryFS::default().into()]);
let storage_provider = VirtualStorageProvider::new(fs);

Original post:

Do you see any value in making VirtualStorageProvider::new much harder to call to encourage use of new_overlay or new_memory? Maybe with like a new_physical and then linting that with clippy?

Originally posted by @hildebrandmw in #700