Correct virtual files placeholder files if needed

Correct virtual files placeholder files if needed

In the past not all files were converted to placeholder files when
converting an existing sync folder to a virtual files folder. Because
some files were not converted to placeholder files, the status would
be wrong on the files. This code makes sure that every file in a
virtual files folder is a placeholder file. It could be removed in the
future.

Signed-off-by: Felix Weilbach <felix.weilbach@nextcloud.com>