qos-test: vhost-user test node

qos-test: vhost-user 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.

Note that this test does not allocate any virtio-net structure,
it's all done by the qtest walking graph mechanism.  Nevertheless,
vhost-user-test is a bit more complex than the other tests, because
it requires more complicated setup of back-ends and thus almost each
test has a slightly different opts.before function.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>