qos-test: virtio-net test node

qos-test: virtio-net test node

Convert tests/virtio-net-test in qgraph test node,
virtio-net-test. This test consumes a virtio-net interface
and checks that its function return the expected values.

Some functions are implemented only for virtio-net-pci, so they
don't consume virtio-net, but virtio-net-pci

Note that this test does not allocate any virtio-net structure,
it's all done by the qtest walking graph mechanism

Signed-off-by: Emanuele Giuseppe Esposito <e.emanuelegiuseppe@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>