[Libguestfs] [PATCH nbdkit] vddk: Detect possible VDDK crash and warn

This is a "meta-workaround" for the VDDK bug described in RHEL-54377 just telling the user where to look for actual workarounds. Matthew, does this only happen when the snapshot parameter is present? I'm not sure if we tested the non-snapshot case and if that crashes too. If so we'd need to remove 'snapshot_moref' from the conditional. Rich.