s390-virtio: clear {used,avail}_event_idx on reset as well

The old s390-virtio transport clears the vring used/avail indices in
the shared area on reset. When we enabled event_idx for virtio-blk, we
noticed that this is not enough: We also need to clear the published
used/avail event indices, or reboot will fail.

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