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: NEric Blake <eblake@redhat.com>

src/util/virstoragefile.c

@@ -1525,7 +1525,7 @@ virStorageFileParseChainIndex(const char *diskTarget,
     if (virStringListLength(strings) != 2)
         goto cleanup;
 
-    if (virStrToLong_ui(strings[1], &suffix, 10, &idx) < 0 ||
+    if (virStrToLong_uip(strings[1], &suffix, 10, &idx) < 0 ||
         STRNEQ(suffix, "]"))
         goto cleanup;