Vfs: Remove VfsDefaults

Vfs: Remove VfsDefaults

That just complicated things. It's ok if Vfs is not a fully abstract
interface class.

The pinstate-in-db methods are instead provided directly on Vfs and
VfsSuffix and VfsOff use them to implement pin states.

The start() method is simply non-virtual and calls into startImpl() for
the plugin-specific startup code.