conf: start testing contents of the new backing chain fields
    
    The testsuite is absolutely essential to feeling comfortable
    about swapping the backing chain structure over to a new format.
    This patch tests the path settings, and demonstrates that the
    correct short name is being passed to the child.
    
    * tests/virstoragetest.c (testStorageChain): Test path.
    (mymain): Update expected data.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>