s390x/cpu hotplug: Fix memory leak

valgrind complains about the following:
==42117== 8 bytes in 1 blocks are definitely lost in loss record 88 of 833
==42117==    at 0x4031AFE: malloc (vg_replace_malloc.c:292)
==42117==    by 0x8022F855: malloc_and_trace (vl.c:2715)
==42117==    by 0x4145569: g_malloc (in /usr/lib64/libglib-2.0.so.0.3400.2)
==42117==    by 0x800F696D: qemu_extend_irqs (irq.c:51)
==42117==    by 0x800F6AF7: qemu_allocate_irqs (irq.c:68)
==42117==    by 0x8029FA4B: irq_cpu_hotplug_init (sclpcpu.c:84)
==42117==    by 0x80297C79: event_realize (event-facility.c:386)
==42117==    by 0x80105071: device_set_realized (qdev.c:693)
[...]

Right it is. Don't drop the pointer of the irq.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Jason J. Herne <jjherne@us.ibm.com>
