s390-ccw.img: Fix sporadic reboot hangs: Initialize next_idx

The current code does not initialize next_idx in the virtio ring.
As the ccw bios will always use guest memory at a fixed location,
this queue might != 0 after a reboot.
Lets make the initialization explicit.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
