CA-419840 improve logging for VDI.forget (#6839)

When a CDR is removed from an ISO SR the corresponding VDI is deleted.
This is not obvious in the logs because some logs refer to the VDI by
reference and others by UUID. Tie this together.

Author: lindig

ocaml/xapi/xapi_vdi.ml

@@ -736,7 +736,9 @@ let pool_introduce = internal_db_introduce
 let db_introduce = internal_db_introduce
 
 let db_forget ~__context ~vdi =
-  debug "db_forget uuid=%s" (Db.VDI.get_uuid ~__context ~self:vdi) ;
+  debug "db_forget uuid=%s ref=%s"
+    (Db.VDI.get_uuid ~__context ~self:vdi)
+    (Ref.string_of vdi) ;
   Db.VDI.destroy ~__context ~self:vdi
 
 let introduce ~__context ~uuid ~name_label ~name_description ~sR ~_type