spapr_rng: disable hotpluggability

It is currently possible to hotplug a spapr_rng device but QEMU crashes
when we try to hot unplug:

ERROR:hw/core/qdev.c:295:qdev_unplug: assertion failed: (hotplug_ctrl)
Aborted

This happens because spapr_rng isn't plugged to any bus and sPAPR does
not provide hotplug support for it: qdev_get_hotplug_handler() hence
return NULL and we hit the assertion.

And anyway, it doesn't make much sense to unplug this device since hcalls
cannot be unregistered. Even the idea of hotplugging a RNG device instead
of declaring it on the QEMU command line looks weird.

This patch simply disables hotpluggability for the spapr-rng class.

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
