spapr: limit numa memory regions by ram size

This makes sure that all NUMA memory blocks reside within RAM or
have zero length.

Reviewed-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
