libvirt users - Mar 2010 - Attach-interface vs attach-device

Working with the two commands there's a noticeable difference in the time
redhat takes to detect interfaces using attach-interface vs attach-device? 
About 5-8 seconds using attach interface, and near instant with attach-device.

That little bit aside, I'm trying to find a way to 'unplug' a VM
without actually removing the interface.

The easy fix is to attach it without adding the tap interface to mybridge, but
attach device and interface do not allow this.

Looking in the source for libvirt I found the InterfacesStartmodeDefFormat
however it seems that virsh doesn't export this option.  Maybe this just
needs to be implemented?

Might anyone have insights as to the best method to pursue?
I'm willing to do the leg work but I'm just starting to look into how
the sources are laid out and that's going to take a bit. Pointers are
appreciated.

Thanks in advance,
Steve