s390x/cpumodel: Add "-cpu max" support

s390x/cpumodel: Add "-cpu max" support

The "max" CPU model behaves like "-cpu host" when KVM is enabled, and like
a CPU with the maximum possible feature set when TCG is enabled.

While the "host" model can not be used under TCG ("kvm_required"), the
"max" model can and "Enables all features supported by the accelerator in
the current host".

So we can treat "host" just as a special case of "max" (like x86 does).
It differs to the "qemu" CPU model under TCG such that compatibility
handling will not be performed and that some experimental CPU features
not yet part of the "qemu" model might be indicated.

These are right now under TCG (see "qemu_MAX"):
- stfle53
- msa5-base
- zpci

This will result right now in the following warning when starting QEMU TCG
with the "max" model:
    "qemu-system-s390x: warning: 'msa5-base' requires 'kimd-sha-512'."

The "qemu" model (used as default in QEMU under TCG) will continue to
work without such warnings. The "max" model in the current form
might be interesting for kvm-unit-tests (where we would e.g. now also
test "msa5-base").

The "max" model is neither static nor migration safe (like the "host"
model). It is independent of the machine but dependends on the accelerator.
It can be used to detect the maximum CPU model also under TCG from upper
layers without having to care about CPU model names for CPU model
expansion.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180725091233.3300-1-david@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
[CH: minor wording changes]
Signed-off-by: Cornelia Huck <cohuck@redhat.com>