Description of problem:
Change vnc password on the fly via "virsh update-device", then shutdown the guest, re-start the guest, when connect the guest windows, the new password still take effect.
As my understand, after the guest is shutdown, the vnc password should be restored to original password for next connection. 

Version-Release number of selected component (if applicable):
libvirt-0.8.1-6.el6.x86_64
qemu-kvm-0.12.1.2-2.54.el6.x86_64
kernel-2.6.32-27.el6.x86_64

How reproducible:
Always

Steps to Reproduce:
1. Define a domain with a vnc password in xml config file
...
<graphics type='vnc' port='-1' autoport='yes' listen='127.0.0.1' keymap='en-us' passwd='123456'/>
...
2. Start the guest, and use virt-viewer to connect the guest windows with "aaabbb" password
3. Change password on the fly.
# cat vnc_passwd.xml 
<graphics type='vnc' port='-1' autoport='yes' listen='127.0.0.1' keymap='en-us' passwd='222222'/>
# virsh update-device winxp vnc_passwd.xml 
Device updated successfully
4. Connect the guest windows successfully with new password "222222"
5. Shutdown the guest.
6. Start the guest again.
7. Try to Connect the guest windows with old password "123456".

  
Actual results:
With old password "123456", connect failed. With new password "222222", connect is successful.

# virsh dumpxml <guestname>
...
<graphics type='vnc' port='5900' autoport='yes' listen='127.0.0.1' keymap='en-us'/>
...

# cat /etc/libvirt/qemu/<guestname>.xml
...
<graphics type='vnc' port='-1' autoport='yes' listen='127.0.0.1' keymap='en-us' passwd='123456'/>
...

# virsh edit <guestname>
...
<graphics type='vnc' port='-1' autoport='yes' listen='127.0.0.1' keymap='en-us' passwd='222222'/>
...

Expected results:
With old password "123456", connect should be successful.

Additional info:
After I restart libvirtd, the original password take effect.

When I try to run this command with "--persistent", but it failed with the following error, I am not sure it is expected.
# virsh update-device winxp vnc_passwd.xml --persistent
error: Failed to update device from vnc_passwd.xml
error: Requested operation is not valid: cannot modify the persistent configuration of a domain