NUMA: Replace MAX_NODES with nb_numa_nodes in for loop

In QEMU, the number of the NUMA nodes is determined by parse_numa_opts().
Then, QEMU uses it for iteration, for example:
  for (i = 0; i < nb_numa_nodes; i++)

However, in memory_region_allocate_system_memory(), it uses MAX_NODES
not nb_numa_nodes.

So, replace MAX_NODES with nb_numa_nodes to keep code consistency and
reduce the loop times.

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Message-Id: <1503387936-3483-1-git-send-email-douly.fnst@cn.fujitsu.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
