util: Fix condition check in virDiskNameToIndex

Use the more common '< 0' rather than the non-zero check.

src/util/virutil.c

@@ -562,7 +562,7 @@ int virDiskNameToIndex(const char *name)
 {
     int idx;
 
-    if (virDiskNameParse(name, &idx, NULL))
+    if (virDiskNameParse(name, &idx, NULL) < 0)
         idx = -1;
 
     return idx;