s390x/sclp: fix missing be conversion

Linux crashes right now if maxmem > mem is specified on the command line.

On s390x, the guest can hotplug memory itself right now - very weird -
and e.g. Fedora 27 will simply add all memory it can when booting.

So now, we have at least the same behavior on TCG and KVM.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20171218224616.21030-3-david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>