conf: tweak chain lookup internals
    
    Thanks to the testsuite, I feel quite confident that this rewrite
    is correct; it gives the same results for all cases except for one.
    I can make the argument that _that_ case was a pre-existing bug:
    when looking up relative names, the lookup is supposed to be
    pegged to the directory that contains the parent qcow2 file.  Thus,
    this resolves the fixme first mentioned in commit 367cd69 (even
    though I accidentally removed the fixme comment early in 74430fe).
    
    * src/util/virstoragefile.c (virStorageFileChainLookup): Depend on
    new rather than old fields.
    * tests/virstoragetest.c (mymain): Adjust test to match fix.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>