snapshot: conf: Use common parsing and formatting functions for source

snapshot: conf: Use common parsing and formatting functions for source

Disk source elements for snapshots were using separate code from our
config parser. As snapshots can be stored on more than just regular
files, we will need the universal parser to allow us to expose a variety
of snapshot disk targets. This patch reuses the config parsers and
formatters to do the job.

This initial support only changes the code without any visible XML
change.