fuzz: only use generic-fuzz targets on oss-fuzz

fuzz: only use generic-fuzz targets on oss-fuzz

The non-generic-fuzz targets often time-out, or run out of memory.
Additionally, they create unreproducible bug-reports. It is possible
that this is resulting in failing coverage-reports on OSS-Fuzz. In the
future, these test-cases should be fixed, or removed.

Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20220623125505.2137534-1-alxndr@bu.edu>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>