s390x: raise CPU hotplug irq after really hotplugged

Let's move it into the machine, so we trigger the IRQ after setting
ms->possible_cpus (which SCLP uses to construct the list of
online CPUs).

This also fixes a problem reported by Thomas Huth, whereby qemu can be
crashed using the none machine

qemu-s390x-softmmu -M none -monitor stdio
-> device_add qemu-s390-cpu

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170928134609.16985-3-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
