numa,spapr: align default numa node memory size to 256MB

Since commit 224245bf ("spapr: Add LMB DR connectors"), NUMA node
memory size must be aligned to 256MB (SPAPR_MEMORY_BLOCK_SIZE).

But when "-numa" option is provided without "mem" parameter,
the memory is equally divided between nodes, but 8MB aligned.
This can be not valid for pseries.

In that case we can have:
$ ./ppc64-softmmu/qemu-system-ppc64 -m 4G -numa node -numa node -numa node
qemu-system-ppc64: Node 0 memory size 0x55000000 is not aligned to 256 MiB

With this patch, we have:
(qemu) info numa
3 nodes
node 0 cpus: 0
node 0 size: 1280 MB
node 1 cpus:
node 1 size: 1280 MB
node 2 cpus:
node 2 size: 1536 MB

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>