Description of problem:
Could not update spice/vnc password to empty for live domain. I think it should take effect to update spice/vnc password from "aabb" to empty for live domain.If virt-manager does not support to change the password to empty, when click 'Apply', it should not succeed.

Version-Release number of selected component (if applicable):
kernel-2.6.32-207.el6.x86_64
qemu-kvm-0.12.1.2-2.196.el6.x86_64
libvirt-0.9.4-19.el6.x86_64
python-virtinst-0.600.0-5.el6.noarch
virt-manager-0.9.0-7.el6.x86_64
virt-viewer-0.4.1-7.el6.x86_64

How reproducible:
always

Steps to Reproduce:

Update vnc password for live domain:

1. Open virt-manager, run a guest successfully with vnc password is "aabb"

2. Open guest details page, click Display VNC, Modify the vnc password to empty. Apply

3. Close the original guest windows, connect guest windows again.
   # virt-viewer <guestname>
   Or quit the whole virt-manager first, then launch virt-manager again, and open the guest details, the password will be informed to fill in.

4. use virt-viewer <guestname>: don't input password, because we changed our password to empty. Then click OK. Password is also "ccdd", did not change.(NOT expected.)
   use virt-manager, don't input password, because we changed our password to empty.Then click "Login",then console shows an error:
   "Error: viewer connection to hypervisor host got refused or disconnected!"(NOT expected.Password should be allowed to input again.It is different with spice graphics.)

Update spice password for live domain:

Steps are same with step 1,2,3 above.

In step4:

use virt-viewer <guestname>: don't input password, because we changed our password to empty. Then click OK. Password is also "ccdd", did not change.(NOT expected.)

   use virt-manager: don't input password, because we changed our password to empty.Then click "Login",then console suggests you to input again. Password is also "ccdd", did not change.(NOT expected.)
  
Actual results:
see above.
Expected results:
see above.

Additional info: