Description of problem:
Hostplug USB to a guest,then shutdown guest -> start guest,fail to hostplug USB to guest.
Shutdown guest,passthrough USB to guest,then start guest -> shutdown guest,after that,fail to start guest again.

Version-Release number of selected component (if applicable):
libvirt-0.9.10-3.el6.x86_64
virt-manager-0.9.0-10.el6.x86_64

How reproducible:
always

Steps to Reproduce:

# virsh nodedev-dumpxml usb_2_1
<device>
  <name>usb_2_1</name>
  <parent>usb_usb2</parent>
  <driver>
    <name>usb</name>
  </driver>
  <capability type='usb_device'>
    <bus>2</bus>
    <device>8</device>
    <product id='0x1625'>DT 101 II</product>
    <vendor id='0x0951'>Kingston</vendor>
  </capability>
</device>

Hotplug:
1.Start a guest.
2.Prepare a USB xml with product + vendor.
#cat hostdev-usb-product.xml 
<hostdev mode='subsystem' type='usb' managed='yes'>
      <source>
              <product id='0x1625'/>
              <vendor id='0x0951'/>
       </source>
</hostdev>

3.#virsh attach-device rhel6.2-new hostdev-usb-product.xml
Device attached successfully
4.Shutdown guest,then start it.

5.Attach the USB device to gust again
# virsh attach-device rhel6.2-new hostdev-usb-product.xml 
error: Failed to attach device from hostdev-usb-product.xml
error: Requested operation is not valid: USB device 002:010 is in use by domain ;�m

Passthrough:
1.Shutdown a guest.
2.Add the USB xml(product+vendor) to guest.
#virsh edit rhel6.2-new
...
<hostdev mode='subsystem' type='usb' managed='yes'>
      <source>
              <product id='0x1625'/>
              <vendor id='0x0951'/>
       </source>
</hostdev>
...
3.Start the guest.Wait for the guest boot successfully,then shutdown it.
4.Start the guest again.
# virsh start rhel6.2-new
error: Failed to start domain rhel6.2-new
error: Requested operation is not valid: USB device 002:010 is in use by domain 

Actual results:
As above

Expected results:
1.The usb can be hotplugged and the guest can be started successfully.
2.Restart libvirtd,the usb can works well.

Additional info:
The USB works well when hotplug/passthrough with bus and device.