CA-388210: SMAPIv3 concurrency: turn on concurrent operations by default

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.

Signed-off-by: Edwin Török <[email protected]>

Author: edwintorok

ocaml/xapi-storage-script/main.ml

@@ -1793,7 +1793,7 @@ let rec diff a 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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