storage: Modify virStorageBackendDiskMakeDataVol logic
    
    Alter the logic such that we only add the volume to the pool once
    we've filled in all the information and cause failure to go to a
    common error: label. Patches to place the @vol into a few hash tables
    will soon "require" that at least the keys (name, target.path, and key)
    be populated with valid data.