pc: add node-id property to CPU

pc: add node-id property to CPU

it will allow switching from cpu_index to property based
numa mapping in follow up patches.

PS:
patch changes default value of CPUState::numa_node from 0
to CPU_UNSET_NUMA_NODE_ID. The only place for x86 that
would affected is monitor's 'infor numa' command which
uses that field. However legacy 0 value is still preserved
by pc_cpu_pre_plug() in this patch if user/numa.c hasn't
set it explicitly, so there is no change in behavior.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1494415802-227633-4-git-send-email-imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>