numa: properly check if numa is supported

numa: properly check if numa is supported

Commit aa57020774b, by mistake used MachineClass::numa_mem_supported
to check if NUMA is supported by machine and also as unrelated change
set it to true for sbsa-ref board.

Luckily change didn't break machines that support NUMA, as the field
is set to true for them.

But the field is not intended for checking if NUMA is supported and
will be flipped to false within this release for new machine types.

Fix it:
 - by using previously used condition
      !mc->cpu_index_to_instance_props || !mc->get_default_cpu_node_id
   the first time and then use MachineState::numa_state down the road
   to check if NUMA is supported
 - dropping stray sbsa-ref chunk

Fixes: aa57020774b690a22be72453b8e91c9b5a68c516
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1576154936-178362-3-git-send-email-imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>