storage: reject negative indices
    
    Commit f22b7899 stumbled across a difference between 32-bit and
    64-bit platforms when parsing "-1" as an int.  Now that we've
    fixed that difference, it's time to fix the testsuite.
    
    * src/util/virstoragefile.c (virStorageFileParseChainIndex):
    Require a positive index.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>