s390x: change the QEMU cpu model to a stripped down z12

s390x: change the QEMU cpu model to a stripped down z12

We are good enough to boot upstream Linux kernels / Fedora 26/27. That
should be sufficient for now.

As the QEMU CPU model is migration safe, let's add compatibility code.
Generate the feature list to reduce the chance of messing things up in the
future.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171208165529.14124-1-david@redhat.com>
[CH: squashed 's390x/cpumodel: make qemu cpu model play with "none" machine'
(20171213132407.5227-1-david@redhat.com) and 's390x/tcg: don't include z13
features in the qemu model' (20171213171512.17601-1-david@redhat.com) into
patch]
Signed-off-by: Cornelia Huck <cohuck@redhat.com>