target/s390x: improve baselining if certain base features are missing

There are certain features that we put into base models, but that are
not relevant for the actual search. The most famous example are
MSA subfunctions that might be disabled on certain real hardware out
there.

While the kvm host model detection will usually detect the correct model
on such machines (as it will in the common case not pass features to check
for into s390_find_cpu_def()), baselining will fall back to a quite old
model just because some MSA subfunctions are missing.

Let's improve that by ignoring lack of these features while performing
the search for a base model.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170720123721.12366-6-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
