s390x/cpu topology: add max_threads machine class attribute

s390x/cpu topology: add max_threads machine class attribute

The S390 CPU topology accepts the smp.threads argument while
in reality it does not effectively allow multthreading.

Let's keep this behavior for machines older than 7.2 and
refuse to use threads in newer machines until multithreading
is really exposed to the guest by the machine.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Message-Id: <20221103170150.20789-3-pmorel@linux.ibm.com>
[thuth: Small fixes to the commit description]
Signed-off-by: Thomas Huth <thuth@redhat.com>