[s390x] Add virtio_net to nic-modules
This adds virtio_net to nic-modules. This is useful both for KVM hypervisors running on s390x as well as system emulation using qemu. Right now debian-installer offers a KVM virtio network interface option in s390-netdevice but the driver is no longer present.
Note: I tried to test this fix but my test VMs do not have sufficient disk space to actually build the kernel. So I am a little \noideadog here, but I'd assume that this should be straightforward.