CA-388210: SMAPIv3 concurrency: turn on concurrent operations by default (#6140)

It is believed that the deadlocks in SMAPIv3 were caused by VDI.copy
operations that attach the same disk RO multiple times in Dom0.
With the previous commits we now use a unique identifier and spawn a
separate qemu-dp process in the SMAPIv3 plugins, which should prevent
the deadlocks and IO errors due to lack of refcounting.

We have also identified the source of deadlocks in xapi-storage-plugins,
as a missing _qmp_disconnect, so it should now be safe to enable this by
default.

Author: edwintorok

ocaml/xapi-storage-script/main.ml

@@ -1791,9 +1791,7 @@ let rec diff a b =
   | a :: aa ->
       if List.mem a b then diff aa b else a :: diff aa b
 
-(* default false due to bugs in SMAPIv3 plugins,
-   once they are fixed this should be set to true *)
-let concurrent = ref false
+let concurrent = ref true
 
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