pc-dimm: validate node property

If user specifies a node number that exceeds the available numa nodes in
emulated system for pc-dimm device, the device will report an invalid _PXM
to OSPM. Fix this by checking the node property value.

Cc: qemu-stable@nongnu.org
Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
