libxl: events: Break out libxl__pipe_nonblock, _close

libxl: events: Break out libxl__pipe_nonblock, _close

Break out the pipe creation and destruction from the poller code
into two new functions libxl__pipe_nonblock and libxl__pipe_close.
Also change direct use of pipe() to libxl_pipe.

No overall functional difference other than minor differences in exact
log messages.

Also move libxl__self_pipe_wakeup and libxl__self_pipe_eatall into the
new pipe utilities section in libxl_event.c; this is pure code motion.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Jim Fehlig <jfehlig@suse.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
--
v3: Mention that we switched pipe() -> libxl_pipe()