s390x/ipl: Fix boot if no bootindex was specified

commit fa92e218 ("s390x/ipl: avoid sign extension") introduced
a regression:

qemu-system-s390x -drive file=image.qcow,format=qcow2
does not boot, the bios states
"No virtio-blk device found!"

adding bootindex=1 does boot.

The reason is that the uint32_t as return value will not do the right
thing for the return -1 (default without bootindex).
The bios itself, will interpret a 64bit -1 as autodetect (but it will
interpret 32bit -1 as ccw device address ff.ff.ffff)
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: qemu-stable@nongnu.org # v2.3.0
Tested-by: NAurelien Jarno <aurelien@aurel32.net>
Reviewed-by: NAurelien Jarno <aurelien@aurel32.net>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>

hw/s390x/ipl.c

@@ -218,7 +218,7 @@ static Property s390_ipl_properties[] = {
  * - -1 if no valid boot device was found
  * - ccw id of the boot device otherwise
  */
-static uint32_t s390_update_iplstate(CPUS390XState *env, S390IPLState *ipl)
+static uint64_t s390_update_iplstate(CPUS390XState *env, S390IPLState *ipl)
 {
     DeviceState *dev_st;
 
@@ -248,7 +248,7 @@ static uint32_t s390_update_iplstate(CPUS390XState *env, S390IPLState *ipl)
 
     return -1;
 out:
-    return ipl->cssid << 24 | ipl->ssid << 16 | ipl->devno;
+    return (uint32_t) (ipl->cssid << 24 | ipl->ssid << 16 | ipl->devno);
 }
 
 int s390_ipl_update_diag308(IplParameterBlock *iplb)