target/s390x: fix pgm irq ilen for stsi

The instruction is 4 bytes long.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170721125609.11117-2-david@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>