S390: Assign default model "virtio" for network interfaces

S390: Assign default model "virtio" for network interfaces

If a network interface model is not specified, libvirt will run
into an unchecked NULL pointer coredump. On the other hand if
the empty model is ignored, a PCI bus address would be generated,
which is not supported by S390.
Since the only valid network type model for S390 is virtio,
we use this as the default value, which is the same for QEMU.

Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>