util: storage: Improve error message when requesting image above 'start'

When a user would specify a backing chain index that is above the start
point libvirt would report a rather unhelpful error:

invalid argument: could not find backing store 1 in chain for 'sub/link2'

This patch adds an explicit check that the index is below start point in
the backing store and reports the following error if not:

invalid argument: requested backing store index 1 is above 'sub/../qcow2' in chain for 'sub/link2'

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1177062

src/util/virstoragefile.c

@@ -1338,6 +1338,15 @@ virStorageFileChainLookup(virStorageSourcePtr chain,
             chain = chain->backingStore;
             i++;
         }
+
+        if (idx && idx < i) {
+            virReportError(VIR_ERR_INVALID_ARG,
+                           _("requested backing store index %u is above '%s' "
+                             "in chain for '%s'"),
+                           idx, NULLSTR(startFrom->path), NULLSTR(start));
+            return NULL;
+        }
+
         *parent = startFrom;
     }