[libvirt-users] attach-interface not permanent?

Hi, I just created a new interface for a running Windows 2008 guest using the attach-interface command which succeeded and I could see the interface in the guest and in the dumpxml output. Then I switched the type to virtio in virt-manager after which I got the message that this change would only appear after the next guest shutdown so I went ahead and shut down the guest. After that the new interface was gone and no longer showed up in the dumpxml output. I'm confused because at no point was it mentioned that the interface would only be added temporarily and the man-page doesn't say anything to that effect either. This happened on a fresh Centos 6.3 system. Any ideas what is going on here? Regards, Dennis