numa: move numa global variable nb_numa_nodes into MachineState

Add struct NumaState in MachineState and move existing numa global
nb_numa_nodes(renamed as "num_nodes") into NumaState. And add variable
numa_support into MachineClass to decide which submachines support NUMA.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Suggested-by: Igor Mammedov <imammedo@redhat.com>
Suggested-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Tao Xu <tao3.xu@intel.com>
Message-Id: <20190809065731.9097-3-tao3.xu@intel.com>
[ehabkost: include hw/boards.h again to fix build failures]
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>