s390x/sclp: mark sclp-cpu-hotplug as non-usercreatable

A TYPE_SCLP_CPU_HOTPLUG device for handling cpu hotplug events
is already created by the sclp event facility. Adding a second
TYPE_SCLP_CPU_HOTPLUG device via -device sclp-cpu-hotplug creates
an ambiguity in raise_irq_cpu_hotplug(), leading to a crash once
a cpu is hotplugged.

To fix this, disallow creating a sclp-cpu-hotplug device manually.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
