I don't know how realistic this is, but it would be cool if libvirt could report in <capabilities> output if the host supports vt-d/amd-vi == pci passthrough. That way apps could have a bit of a chance letting users know if it will even work.

Also according to this:

http://www.linux-kvm.org/page/How_to_assign_devices_with_VT-d_in_KVM

Plain PCI devices (vs. PCI-e) kinda suck for passthrough, as do devices that don't support MSI. If there was some way to report that in <nodedev> output that would be cool too (though probably material for another bug report).