libxl: events: Make libxl__async_exec_* pass caller an rc

libxl: events: Make libxl__async_exec_* pass caller an rc

The internal user of libxl__async_exec_start et al now gets an rc as
well as the process's exit status.

For now this is always either 0 or ERROR_FAIL, but with ao
abort requests this will possibly be ABORTED or TIMEDOUT too.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
---
v4: Improve doc comment as suggested by Ian C.
v2: New patch due to rebause; v1 had changes to device_hotplug_*
     scripts instead.
    Callback now gets unambiguous information about error situation:
     previously, if only thing that went wrong was that child died
     badly, rc would be FAILED, which was unambigously; now rc=0.
    Add a comment document the meaning of the rc and status parameters
     to the callback.