Migration: Ensure destination directories exist when moving RRDs

If you have not run collectd 5 before, then when type instances
are migrated to plugin instances the destination directories
may not already exist. For example:

mv "./test/df/df-mnt-dlna.rrd" "./test/df-mnt-dlna/df.rrd"

This one-liner does a mkdir -p of the destination before
attempting to move the file.