spapr: Don't clamp RMA to 16GiB on new machine types

In spapr_machine_init() we clamp the size of the RMA to 16GiB and the
comment saying why doesn't make a whole lot of sense.  In fact, this was
done because the real mode handling code elsewhere limited the RMA in TCG
mode to the maximum value configurable in LPCR[RMLS], 16GiB.

But,
 * Actually LPCR[RMLS] has been able to encode a 256GiB size for a very
   long time, we just didn't implement it properly in the softmmu
 * LPCR[RMLS] shouldn't really be relevant anyway, it only was because we
   used to abuse the RMOR based translation mode in order to handle the
   fact that we're not modelling the hypervisor parts of the cpu

We've now removed those limitations in the modelling so the 16GiB clamp no
longer serves a function.  However, we can't just remove the limit
universally: that would break migration to earlier qemu versions, where
the 16GiB RMLS limit still applies, no matter how bad the reasons for it
are.

So, we replace the 16GiB clamp, with a clamp to a limit defined in the
machine type class.  We set it to 16 GiB for machine types 4.2 and earlier,
but set it to 0 meaning unlimited for the new 5.0 machine type.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

include/hw/ppc/spapr.h

@@ -126,6 +126,7 @@ struct SpaprMachineClass {
     bool pre_4_1_migration; /* don't migrate hpt-max-page-size */
     bool linux_pci_probe;
     bool smp_threads_vsmt; /* set VSMT to smp_threads by default */
+    hwaddr rma_limit;          /* clamp the RMA to this size */
 
     void (*phb_placement)(SpaprMachineState *spapr, uint32_t index,
                           uint64_t *buid, hwaddr *pio,