libxl: event: Fix possible hang with libxl_osevent_beforepoll

libxl: event: Fix possible hang with libxl_osevent_beforepoll

If the application uses libxl_osevent_beforepoll, a similar hang is
possible to the one described and fixed in
   libxl: event: Fix hang when mixing blocking and eventy calls
Application behaviour would have to be fairly unusual, but it
doesn't seem sensible to just leave this latent bug.

We fix the latent bug by waking up the "poller_app" pipe every time we
add osevents.  If the application does not ever call beforepoll, we
write one byte to the pipe and set pipe_nonempty and then we ignore
it.  We only write another byte if beforepoll is called again.

Normally in an eventy program there would only be one thread calling
libxl_osevent_beforepoll.  The effect in such a program is to
sometimes needlessly go round the poll loop again if a timeout
callback becomes interested in a new osevent.  We'll fix that in a
moment.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Tested-by: George Dunlap <george.dunlap@citrix.com>
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v2: New addition to correctness arguments in libxl_event.c comment.