API|Savegame Converter: HOOK_SAVEGAME_CONVERT arguments are now VFS paths

Both the input file and the output folder are now described using
virtual file system paths. It is the job of the conversion plugin to
determine whether these point to native locations in the local file
system and if necessary, alter their behavior accordingly.

The Savegame Converter plugin, which delegates the conversion using
the command line Savegame Tool, now resolves these into native paths
before attempting a conversion.