cpu: No need to zero-initialize CPUState::numa_node

QOM objects are already zero-filled when instantiated, there's no need
to explicitly set numa_node to 0.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
