test: replace qemu nic model e1000 by virtio

test: replace qemu nic model e1000 by virtio

TEST-72-NBD fails on s390x with:

```
qemu-system-s390x: warning: hub port hub0port0 has no peer
qemu-system-s390x: warning: hub 0 with no nics
qemu-system-s390x: warning: netdev hub0port0 has no peer
qemu-system-s390x: warning: requested NIC (#net046, model e1000) was not created (not supported by this machine?)
```

The test uses the e1000 model, but s390x does not have it:

```
$ qemu-system-s390x -nic model=help
Available NIC models:
virtio
```

So replace the e1000 model by virtio, which is supported by most
architectures.

Fixes #953