Description of problem:
Trying to run a vm with the 3rd virtio-scsi disk will fail.

Version-Release number of selected component (if applicable):
current master

How reproducible:
100%

Steps to Reproduce:
1. create a vm with 2 virtio-scsi disks
2. run the vm - wait till it populates the vm device (you should see it under vm device tab - 2 disks with 2 different addresses:
  {type=drive, bus=0, controller=0, target=0, unit=0} 
AND
  {type=drive, bus=0, controller=0, target=0, unit=2})

3. shutdown the vm
4. add the 3rd virtio-scsi disk
5. run vm

Actual results:
fails with:
Exit message: unsupported configuration: Found duplicate drive address for disk with target name 'sdb' controller='0' bus='0' target='0' unit='2'.

Expected results:
the 3rd disk get the proper unit 3 and so on.

Additional info:
Possibly a virt bug and not storage because of the vm structure creation.