xapi_vm_migrate: Avoid duplicate, overly-strict CBT check on VDIs (#6405)

There is already a call to `assert_can_migrate_vdis` present in
`assert_can_migrate`, which checks that none of the VDIs that *are going
to be moved* have CBT enabled. There is no need to additionally check
that none of the VDIs *in general* have CBT enabled.

Some clients, like XenOrchestra, will turn off CBT on VDIs and retry
migration after getting the `VDI_CBT_ENABLED` error on live migration.
Dropping this overly strict check allows not stripping CBT when VDI will
not be moved (when it's on a shared SR).
In addition, during rolling pool upgrades, disabling CBT is not allowed,
hence the live migration operation wouldn't be able to continue.
Avoiding the strict check fixes that as well.

Closes #6400

Author: psafont

ocaml/xapi/xapi_vm_migrate.ml

@@ -1778,14 +1778,6 @@ let assert_can_migrate ~__context ~vm ~dest ~live:_ ~vdi_map ~vif_map ~options
   let vbds = Db.VM.get_VBDs ~__context ~self:vm in
   let vms_vdis = List.filter_map (vdi_filter __context true) vbds in
   check_vdi_map ~__context vms_vdis vdi_map ;
-  (* Prevent SXM when the VM has a VDI on which changed block tracking is enabled *)
-  List.iter
-    (fun vconf ->
-      let vdi = vconf.vdi in
-      if Db.VDI.get_cbt_enabled ~__context ~self:vdi then
-        raise Api_errors.(Server_error (vdi_cbt_enabled, [Ref.string_of vdi]))
-    )
-    vms_vdis ;
   (* operations required for migration *)
   let required_src_sr_operations = Smint.Feature.[Vdi_snapshot; Vdi_mirror] in
   let required_dst_sr_operations =
@@ -1919,6 +1911,9 @@ let assert_can_migrate ~__context ~vm ~dest ~live:_ ~vdi_map ~vif_map ~options
     )
   ) ;
   (* check_vdi_map above has already verified that all VDIs are in the vdi_map *)
+  (* Previously there was also a check that none of the VDIs have CBT enabled.
+     This is unnecessary, we only need to check that none of the VDIs that
+     *will be moved* have CBT enabled. *)
   assert_can_migrate_vdis ~__context ~vdi_map
 
 let assert_can_migrate_sender ~__context ~vm ~dest ~live:_ ~vdi_map:_ ~vif_map:_