s390/ipl: Fix spurious errors in virtio

s390/ipl: Fix spurious errors in virtio

With the ccw ipl code sometimes an error message like
"virtio: trying to map MMIO memory" or
"Guest moved used index from %u to %u" appeared. Turns out
that the ccw bios did not zero out the vring, which might
cause stale values in avail->idx and friends, especially
on reboot.

Lets zero out the relevant fields. To activate the patch we
need to rebuild s390-ccw.img as well.

Signed-off-by: Christian Borntraeger <[email protected]>
Message-id: 1369309901[email protected]
Signed-off-by: Anthony Liguori <[email protected]>