libxl: Use libxentoollog in preference to libxenctrl if available.

libxl: Use libxentoollog in preference to libxenctrl if available.

Upstream Xen is in the process of splitting the (stable API) xtl_*
interfaces out from the (unstable API) libxenctrl library and into a
new (stable API) libxentoollog.

In order to be compatible with Xen both before and after this
transition check for xtl_createlogger_stdiostream in a libxentoollog
library and use it if present. If it is not present assume it is in
libxenctrl.

Compile tested on Xen 4.6 and a development tree with the split in
place.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
---
I'm waiting on applying the upstream change until downstreams are
prepared for this. The latest upstream patch is
http://lists.xen.org/archives/html/xen-devel/2015-12/msg00454.html
which had to be reverted because I had somehow not properly checked if
libvirt used this interface
http://lists.xen.org/archives/html/xen-devel/2015-12/msg01153.html

It might be nice to get this into 1.3.0 so that supports Xen 4.7 out
of the box? Not sure what the libvirt stable backport policy is but it
might also be good to eventually consider it for that?