libxl: Comment cleanups

libxl: Comment cleanups

* Add two comments in libxl_remus_disk_drbd documenting buggy handling
  of the hotplug script exit status.

* Add a section heading for async exec in libxl_aoutils.c

* Mention the right function name (libxl__ev_child_fork, not
  libxl__ev_fork) in libxl_internal.h

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Yang Hongyang <yanghy@cn.fujitsu.com>
CC: Wen Congyang <wency@cn.fujitsu.com>
CC: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>