target-s390x: correctly initialize ext interrupt queue

env->ext_index should be initialized to -1 to mark the external
interrupt queue as emtpy. This should not be done in s390_cpu_initfn
as all the interrupt fields are later reset to 0 by the memset in
s390_cpu_initial_reset or s390_cpu_full_reset. Move the initialization
there.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Alexander Graf <agraf@suse.de>
