vfs: Separate vfs availability from new-files-virtual

vfs: Separate vfs availability from new-files-virtual

This helps support 2.5 settings where there are virtual files in the
tree but new files aren't created virtual.

It's also a prelude for #6815

There's currently no way of
- upgrading vfs plugins (a silent suffix->winvfs upgrade is attempted
  once only, when moving to master)
- disabling vfs capabilities outright