Currently, the fstab backend only lists NFS and SMB share, although
the majority of the code could handle any other file system, e.g.
encfs.fuse used for Vaults.

Fix up description to make it more generic, and restructure
devicesFromQuery() to allow extension for other Solid interfaces. The
apparent code duplication will go away as soon as e.g. the
StorageVolume interface has to be handled.