fuzz: avoid double-fetches by default

The generic fuzzer can find double-fetch bugs. However:
* We currently have no good way of producing qemu-system reproducers for
  double-fetch bugs. Even if we can get developers to run the binary-blob
  reproducers with the qemu-fuzz builds, we currently don't have a minimizer for
  these reproducers, so they are usually not easy to follow.
* Often times the fuzzer will provide a reproducer containing a
  double-fetch for a bug that can be reproduced without double-fetching.

Until we find a way to build nice double-fetch reproducers that
developers are willing to look at, lets tell OSS-Fuzz to avoid
double-fetches.

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Message-Id: <20201202164214.93867-1-alxndr@bu.edu>
Signed-off-by: Thomas Huth <thuth@redhat.com>