numa: Rename set_numa_modes() to numa_post_machine_init()

This function does some initialization that needs to be done after
machine init. The function may be eventually removed if we move the
CPUState.numa_node initialization to the CPU init code, but while the
function exists, lets give it a name that makes sense.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>