libxl: ao: Provide manip_refcnt

libxl: ao: Provide manip_refcnt

Previously we used in_initiator to stop the ao being freed while we
were still in the initiator function (which would result in the
initiator's call to libxl__ao_inprogress accessing the ao after it had
been freed).

We are going to introduce a new libxl entrypoint which finds, and
operates on, ongoing aos.  This function needs the same protection,
and might even end up running on the same ao multiple times
concurrently.

So do this with reference counting instead, with a new variable
ao->manip_refcnt.

We keep ao->in_initiator because that allows us to keep some useful
asserts about the sequencing of libxl__ao_inprogress, etc.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
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v5: Fix typo in commit message.
v3: Add a missing space.
    Mention locking in the comment.