xapi: Wrap {vhd,qcow}-tool read_header invocation in a thread

[last-genius]

This allows it to proceed in parallel with the parse side, not deadlocking on the filled pipe.

[edwintorok]

Looks plausible, although there is some potential for some code cleanup in the future: those 2 snippets of code look almost identical.

I'll try creating a build and running some tests.

[lindig]

Looks safe to me.

[edwintorok]

For testing I'm using this commit and it reproduces the vm-export hang (without this PR):

commit d8b425c6315582ab4a4538320cb4ee37b64c406e (HEAD -> private/edvint/test-vhd, origin/private/edvint/test-vhd)
Author: Edwin Török <edwin.torok@citrix.com>
Date:   Mon Feb 9 15:43:10 2026 +0000

    TEST DO NOT MERGE: make JSON bigger

diff --git a/ocaml/vhd-tool/src/impl.ml b/ocaml/vhd-tool/src/impl.ml
index 530c915b8e..6aa73ce3cc 100644
--- a/ocaml/vhd-tool/src/impl.ml
+++ b/ocaml/vhd-tool/src/impl.ml
@@ -1170,6 +1170,7 @@ let stream_t common args ?(progress = no_progress_bar) () =

 let read_headers common source =
   let path = [Filename.dirname source] in
+  print_string (String.make 128000 ' ');

[edwintorok]

With this PR export succeeds (atlhough not sure about error handling from the tool itself, but that'll become obvious if we fail to parse the JSON, and there is a fallback there to read the entire vhd instead then)