nit: Add FIXME to image volume cache

The image volume cache now has logic to clone across pools based on
the value reported by the driver.
There is a case that was missed which is added as a FIXME in this
patch.

Change-Id: I34d7b6788290669d687f9c47f54db084442a7ee3

cinder/image/cache.py

@@ -62,6 +62,11 @@ class ImageVolumeCache(object):
             return {'cluster_name': volume_ref.cluster_name}
         if not self.clone_across_pools:
             return {'host': volume_ref.host}
+        # FIXME(whoami-rajat): If we have two cinder backends pointing to
+        # two different storage arrays, this logic will allow the operation
+        # to proceed to clone across two storage arrays which will fail
+        # eventually. We should at least filter with the hostname in the
+        # given host value hostname@backend#pool.
         return {}
 
     def get_entry(self,