conf: test backing chain lookup
    
    I realized that we had no good test coverage of looking up a
    name from within a backing chain, even though code like
    block-commit is relying on it.
    
    * tests/virstoragetest.c (testStorageLookup): New function.
    (mymain): New tests.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>