VLCMetadataService: Ensure the instance will outlive the preparsing

Since the instance isn't held by the media, the code assumes the
instance will outlive the preparse request, which was not guaranteed by
the code since another thread could have concurrently replaced the
instance
10 jobs for !727 with hold_instance_during_preparse in 4 minutes and 57 seconds (queued for 3 seconds)
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