libxl: get_reaper_lock_and_uid: Document fd handling

libxl: get_reaper_lock_and_uid: Document fd handling

Coverity understandably complains that get_reaper_lock_and_uid leaks
the fd and hence open-file.  But this is intentional: the lock becomes
owned by the child process as a whole, which is entirely the property
of libxl.

(The coding style here in this subprocess is a bit anomalous but it's
probably not worth it to convert get_reaper_lock_and_uid to `goto out'
style and have it explicitly return the fd number.)

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>