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: Christian 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: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
(cherry picked from commit 6efd2c2a)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>