Fix a wrong comparison in _RecursiveAssembleBD

We want to prevent sending too many 'None' children to a device.
However, the test as it is today is wrong, as we want to test the
situation after adding a new child, and not before. This patch fixes
this by testing greater-or-equal instead of just greater.

Reviewed-by: imsnah

lib/backend.py

@@ -730,7 +730,7 @@ def _RecursiveAssembleBD(disk, owner, as_primary):
       try:
         cdev = _RecursiveAssembleBD(chld_disk, owner, as_primary)
       except errors.BlockDeviceError, err:
-        if children.count(None) > mcn:
+        if children.count(None) >= mcn:
           raise
         cdev = None
         logger.Debug("Error in child activation: %s" % str(err))