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: NIan Campbell <ian.campbell@citrix.com>