virtio-net-failover test tries several device combinations that produces
some expected warnings.
These warning can be confusing, so we disable them during the qtest
sequence.

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <20211220145314.390697-1-lvivier@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
[thuth: Fix memory leak by using error_free()]
Signed-off-by: Thomas Huth <thuth@redhat.com>