conf: Set defaultFormat if no storage source XML present
    
    While parsing if the storage source is not present, then a defaultFormat
    was not set. This could lead to oddities such as seeing "unknown" format
    in output for the "logical" pool even though the only format the pool could
    support would be "lvm2".
    
    This does "put a label" on other pool defaults as follows:
    
       File System: FS_AUTO
       Network File System: NETFS_AUTO
       Disk: UNKNOWN
    
    Each of which is the "0" value for their respective pools and thus
    would be no "real" change.