libvirt users - Mar 2014 - Attach / detach problem for passthrough/SR-IOV in libvirt

Hi all
I just encountered a problem in libvirt while was trying to attach and 
detach a sr-iov vf to my VM

The version of my libvirt is 1.1.1-0ubuntu8.5

I tried to attach the device using following xml

<interface type='hostdev' managed='yes'>
     <mac address='5c:01:fd:12:34:58'/>
     <source>
         <address type='pci' domain='0x0000'
bus='0x04' slot='0x01'
function='0x1'/>
     </source>
</interface>

When i attached device it worked properly and i managed to ping another 
host.

virsh # attach-device t5 1.xml
Device attached successfully

  But when i detached it still i had network traffic in my VM but 
libvirt (virsh) said that

virsh # detach-device t5 1.xml
Device detached successfully

The next time i tried to attach it libvirt said that

virsh # attach-device t5 1.xml
error: Failed to attach device from 1.xml
error: Requested operation is not valid: PCI device 0000:04:01.1 is in 
use by domain t5

I tried it with different VMs and different guest OSs that the same 
happend.

The same action was done with another host running libvirt version

Installed: 1.0.2-0ubuntu11.13.04.5~cloud1
  Candidate: 1.0.2-0ubuntu11.13.04.5~cloud1

  and it worked pretty OK. No problem.

Do you have any idea what is wrong?

More information:
Kernel : 3.8.0-37-generic

  /etc/libvirt/qemu.conf :  security_driver = "none”

root@compute01:~# ethtool -i eth5
driver: bnx2x
version: 1.78.58
firmware-version: bc 7.4.22 phy 1.34
bus-info: 0000:04:00.1
supports-statistics: yes
supports-test: yes
supports-eeprom-access: yes
supports-register-dump: yes