[libvirt] attach device on a defined (non-active) vm

It seems that the attach device function really 'attaches' a (live) device to a non-active domain. Is this by design? Otherwise I would not have a clue how to interpreted this message: libvir: Xen Daemon error : POST operation failed: (xend.err 'Device /dev/xvdp (51952, tap) is already connected.') (when I try to start the domain) So what I did; I have created an empty domain without any devices. Loaded that and added a disk. Stefan