Correct a check for capacity arg of storageVolumeResize()

Lets say I got a volume with '1G' allocation and '10G' capacity. The
available space in the parent pool is '5G'. With the current check for
overcapacity, I can only try to resize to <= '6G'. You see the problem?

src/storage/storage_driver.c

@@ -1758,8 +1758,8 @@ storageVolumeResize(virStorageVolPtr obj,
         goto out;
     }
 
-    if (abs_capacity > vol->allocation + pool->def->available) {
-        virStorageReportError(VIR_ERR_INVALID_ARG,
+    if (abs_capacity > vol->capacity + pool->def->available) {
+        virStorageReportError(VIR_ERR_OPERATION_FAILED,
                               _("Not enough space left on storage pool"));
         goto out;
     }