spapr: Refactor spapr_populate_memory() to allow memoryless nodes

Current QEMU does not support memoryless NUMA nodes, however
actual hardware may have them so it makes sense to have a way
to emulate them in QEMU. This prepares SPAPR for that.

This moves 2 calls of spapr_populate_memory_node() into
the existing loop over numa nodes so first several nodes may
have no memory and this still will work.

If there is no numa configuration, the code assumes there is just
a single node at 0 and it has all the guest memory.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
