Description of problem:
It is not possible to add a new hardware to a virt guest after PCI device for eth0 has been assigned already to a virt guest. The system is configure for PCI device assignement [1], so far untested on a system without active VT-d extensions.

Version-Release number of selected component (if applicable):
virt-manager-0.8.6-4.el6.noarch
libvirt-0.8.7-18.el6.x86_64

How reproducible:
always

Steps to Reproduce:
1. Configure system for PCI device assignement by enabling VT-d extensions [1]
2. Add PCI device for eth0 to a virt guest (details -> Add Hardware ...)
3. Try to add another hadware
  
Actual results:
Not able to "add hardware" to a virt guest.
Error messages in /var/log/messages:

Jul 12 08:58:03 dhcp-25-23 libvirtd: 08:58:03.346: 1935: error : interfaceGetXMLDesc:355 : internal error could not get interface XML description (netcf: NETLINK socket operation failed - couldn't find ifindex for interface `eth0`)

Expected results:
Able to "add hardware" to a virt guest.

Additional info:

http://documentation-stage.bne.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/6/html-single/Virtualization/index.html#chap-Virtualization-PCI_passthrough