CA-405628: unmount/detach PVS cache VDI before destroying (#6266)

The pvsproxy daemon normally unmounts and removes the locally attached
VBD for the cache VDI when needed. We do it as well in
`PVS_cache_storage.destroy`, just in case pvsproxy did not manage to,
for whatever reason. This should avoid any VDI_IN_USE errors.

ocaml/xapi/pvs_cache_vdi.ml

@@ -79,11 +79,24 @@ let get_or_recreate_vdi ~__context ~self =
           vdi
   )
 
+let ensure_vdi_is_closed ~__context vdi =
+  (* The pvsproxy daemon normally unmounts and removes the locally attached VBD
+     for the cache VDI. Here we do it as well, just in case pvsproxy did not
+     manage to, for whatever reason. *)
+  try
+    let uuid = Db.VDI.get_uuid ~__context ~self:vdi in
+    let _ : string =
+      Helpers.call_script !Xapi_globs.pvsproxy_close_cache_vdi [uuid]
+    in
+    ()
+  with _ -> () (* call_script will log anything interesting already *)
+
 let destroy_vdi ~__context ~self =
   match get_vdi ~__context ~self with
   | None ->
       () (* The VDI doesn't exist anymore; nothing to do. *)
   | Some vdi ->
+      ensure_vdi_is_closed ~__context vdi ;
       Helpers.call_api_functions ~__context (fun rpc session_id ->
           Client.Client.VDI.destroy ~rpc ~session_id ~self:vdi
       )

ocaml/xapi/xapi_globs.ml

@@ -939,6 +939,8 @@ let dracut = ref "/usr/bin/dracut"
 
 let udevadm = ref "/usr/sbin/udevadm"
 
+let pvsproxy_close_cache_vdi = ref "/opt/citrix/pvsproxy/close-cache-vdi.sh"
+
 let yum_repos_config_dir = ref "/etc/yum.repos.d"
 
 let remote_repository_prefix = ref "remote"
@@ -1892,6 +1894,10 @@ module Resources = struct
       , "Execute during PIF unplug to get the lun devices related with the \
          ether interface of the PIF"
       )
+    ; ( "pvsproxy_close_cache_vdi"
+      , pvsproxy_close_cache_vdi
+      , "Path to close-cache-vdi.sh"
+      )
     ]
 
   let essential_files =