s390/virtio: change maintainership

Halil is doing a lot more work in the virtio area on s390 than I
do. Let's reflect the reality in the maintainers file.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>