libxl: event: Move poller pipe emptying to the end of afterpoll

libxl: event: Move poller pipe emptying to the end of afterpoll

This seems neater.  It doesn't have any significant effect because:

The poller fd wouldn't be emptied by time_occurs.  It would only be
woken by time_occurs as a result of an ao completing, or by
libxl__egc_ao_cleanup_1_baton.  But ...1_baton won't be called in
between (for one thing, this would violate the rule of not still
having the active caller when ...1_baton is called).

While discussing this patch, I noticed that there is a possibility (in
libxl in general) that poller_put might be called on a woken poller.
It would probably be sensible at some point to make poller_get empty
the pipe, at least if the pipe_nonempty flag is set.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Tested-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
---
v3: Completely revised commit message; now we think this is just
    cleanup.