numa: Introduce MachineClass::auto_enable_numa for implicit NUMA node

numa: Introduce MachineClass::auto_enable_numa for implicit NUMA node

Add MachineClass::auto_enable_numa field. When it is true, a NUMA node
is expected to be created implicitly.

Acked-by: David Gibson <[email protected]>
Suggested-by: Igor Mammedov <[email protected]>
Suggested-by: Eduardo Habkost <[email protected]>
Reviewed-by: Igor Mammedov <[email protected]>
Signed-off-by: Tao Xu <[email protected]>
Message-Id: <20190905083238[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>