s390x/sclpconsole-lm: Fix and simplify irq setup

valgrind complains about a memory leak in irq setup of sclpconsole:

==42117== 8 bytes in 1 blocks are definitely lost in loss record 89of 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 0x800F5685: console_init (sclpconsole.c:235)
==42117==    by 0x80297C79: event_realize (event-facility.c:386)
==42117==    by 0x80105071: device_set_realized (qdev.c:693)
==42117==    by 0x801CDC4B: property_set_bool (object.c:1337)
 ==42117==    by 0x801CBD7F: object_property_set (object.c:819)
[...]

We dont need the indirection of an qemu irq to inject an slcp interrupt.
Fixes a valgrind error and makes the code simpler.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Heinz Graalfs <graalfs@linux.vnet.ibm.com>
