pc-dimm: Don't check dimm->node when there is non-NUMA config

It should not break memory hotplug feature if there is non-NUMA option.

This patch would also allow to use pc-dimm as replacement for initial memory
for non-NUMA configs.

Note: After this patch, the memory hotplug can work normally for Linux guest OS
when there is non-NUMA option and NUMA option. But not support Windows guest OS
to hotplug memory with no-NUMA config, actully, it's Windows limitation.

Reviewed-By: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>