Description of problem:
When I tried to add a direct lun as a VM disk, I found that it can only be added once. After I removed it, I can't see the lun again after click the button 'login'
I was using iscsi as storage server.

Version-Release number of selected component (if applicable):
ovirt-engine-backend-3.1.0_0001-1.8.fc17.noarch

How reproducible:
always


Steps to Reproduce:
1. click "Disks" and then click "Add"
2. check in "external"
3. click "discoery targets", type iscsi server ip address and click "discover"
4. click "login"
  
Actual results:
The first time I can see the lun to be added after login, so it can be added successfully. But after it's removed, I can't see it anymore after login.


Expected results:
Can add direct lun again after it's removed.


Additional info:
I found the lun still exists in the database table "luns" after it's removed from disk. But no entry is found in the table "disk_lun_map". I am not sure if it's related.