s390/ipl: Fix boot order

The latest ipl code adaptions collided with some of the virtio
refactoring rework. This resulted in always booting the first
disk. Let's fix booting from a given ID.
The new code also checks for command lines without bootindex to
avoid random behaviour when accessing dev_st (==0).

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>

hw/s390x/ipl.c

diff --git a/hw/s390x/ipl.c b/hw/s390x/ipl.c
index 0aeb003..d69adb2 100644
--- a/hw/s390x/ipl.c
+++ b/hw/s390x/ipl.c
@@ -154,17 +154,19 @@
     env->psw.mask = IPL_PSW_MASK;
 
     if (!ipl->kernel) {
-        /* booting firmware, tell what device to boot from */
+        /* Tell firmware, if there is a preferred boot device */
+        env->regs[7] = -1;
         DeviceState *dev_st = get_boot_device(0);
-        VirtioCcwDevice *ccw_dev = (VirtioCcwDevice *) object_dynamic_cast(
-                OBJECT(&(dev_st->parent_obj)), "virtio-blk-ccw");
+        if (dev_st) {
+            VirtioCcwDevice *ccw_dev = (VirtioCcwDevice *) object_dynamic_cast(
+                OBJECT(qdev_get_parent_bus(dev_st)->parent),
+                TYPE_VIRTIO_CCW_DEVICE);
 
-        if (ccw_dev) {
-            env->regs[7] = ccw_dev->sch->cssid << 24 |
-                           ccw_dev->sch->ssid << 16 |
-                           ccw_dev->sch->devno;
-        } else {
-            env->regs[7] = -1;
+            if (ccw_dev) {
+                env->regs[7] = ccw_dev->sch->cssid << 24 |
+                               ccw_dev->sch->ssid << 16 |
+                               ccw_dev->sch->devno;
+            }
         }
     }