MAINTAINERS: separate section for s390 virtio drivers

The s390-specific virtio drivers have probably more to do with virtio
than with kvm today; let's move them out into a separate section to
reflect this and to be able to add relevant mailing lists.

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

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>