libxl: New API for providing OS events to libxl

libxl: New API for providing OS events to libxl

We provide a new set of functions and related structures
  libxl_osevent_*
which are to be used by event-driven applications to receive
information from libxl about which fds libxl is interested in, and
what timeouts libxl is waiting for, and to pass back to libxl
information about which fds are readable/writeable etc., and which
timeouts have occurred.  Ie, low-level events.

In this patch, this new machinery is still all unused.  Callers will
appear in the next patch in the series, which introduces a new API for
applications to receive high-level events about actual domains etc.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>