Description of problem:
No error when Hotplug/Coldplug duplicate USB device with bus + device

Version-Release number of selected component (if applicable):
libvirt-0.9.10-2.el6.x86_64

How reproducible:
always

Steps to Reproduce:
[root@zhpeng ~]# lsusb
Bus 001 Device 004: ID 0951:1625 Kingston Technology DataTraveler 101 II

prepare usb device xml:
 <hostdev mode='subsystem' type='usb' managed='yes'>
          <source>
                                  <address bus='1' device='4'/>
          </source>
</hostdev>

# virsh start kvm1
# virsh attach-device kvm1 usb1.xml
Device attached successfully
# virsh attach-device kvm1 usb1.xml
Device attached successfully
# virsh attach-device kvm1 usb1.xml
Device attached successfully

# virsh dumpxml kvm1
...
   <hostdev mode='subsystem' type='usb' managed='yes'>
      <source>
        <address bus='1' device='4'/>
      </source>
      <alias name='hostdev0'/>
    </hostdev>
    <hostdev mode='subsystem' type='usb' managed='yes'>
      <source>
        <address bus='1' device='4'/>
      </source>
      <alias name='hostdev1'/>
    </hostdev>
    <hostdev mode='subsystem' type='usb' managed='yes'>
      <source>
        <address bus='1' device='4'/>
      </source>
      <alias name='hostdev2'/>
    </hostdev>
...

Actual results:
There is no error

Expected results:
Error shows and stop user do this

Additional info: