tools/ocaml: Release the global lock during some hypercalls

tools/ocaml: Release the global lock during some hypercalls

Since libxc is re-entrant, there is no need for the OCaml bindings to
prevent more than one thread from entering libxc concurrently.

Previously, the OCaml bindings had prevented re-entrancy by not using
caml_{enter,leave}_blocking_section in the C stubs. The absence of
these calls meant that the global lock remained held during
hypercalls. This caused multi-threaded applications to completely lock
up during long-running hypercalls.

Calls to these functions were present but commented out in the OCaml
bindings some years ago when libxc was not fully re-entrant.

Instead, we now do call caml_{enter,leave}_blocking_section in all the
places it used to be commented out, meaning that the global lock is
released during those hypercalls. We also no longer assert the
XC_OPENFLAG_NON_REENTRANT flag when calling xc_interface_open because
the caller no longer does re-entrancy prevention at those places.

This patch has now gone through a XenRT nightly test; no problems were
observed.

Signed-off-by: Jonathan Davies <jonathan.davies@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>