spapr: Don't allow memory hotplug to memory less nodes

Currently PowerPC kernel doesn't allow hot-adding memory to memory-less
node, but instead will silently add the memory to the first node that has
some memory. This causes two unexpected behaviours for the user.

- Memory gets hotplugged to a different node than what the user specified.
- Since pc-dimm subsystem in QEMU still thinks that memory belongs to
  memory-less node, a reboot will set things accordingly and the previously
  hotplugged memory now ends in the right node. This appears as if some
  memory moved from one node to another.

So until kernel starts supporting memory hotplug to memory-less
nodes, just prevent such attempts upfront in QEMU.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
