qemu: ppc64: Align memory sizes to 256MiB blocks
    
    For some machine types ppc64 machines now require that memory sizes are
    aligned to 256MiB increments (due to the dynamically reconfigurable
    memory). As now we treat existing configs reasonably in regards to
    migration, we can round all the sizes unconditionally. The only drawback
    will be that the memory size of a VM can potentially increase by
    (256MiB - 1byte) * number_of_NUMA_nodes.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1249006