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 <9be92b3dbbd6611e3e4c9209fb3d04b4919e7cca@citrix.com>

VenkatDatta/libvirt,nertpinx/libvirt,eskultety/libvirt,nertpinx/libvirt,olafhering/libvirt,rlaager/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,zippy2/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,taget/libvirt,andreabolognani/libvirt,libvirt/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,jfehlig/libvirt,jardasgit/libvirt,crobinso/libvirt,crobinso/libvirt,crobinso/libvirt,rlaager/libvirt,nertpinx/libvirt,libvirt/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,eskultety/libvirt,jardasgit/libvirt,zippy2/libvirt,zippy2/libvirt,olafhering/libvirt,datto/libvirt,olafhering/libvirt,rlaager/libvirt,jfehlig/libvirt,rlaager/libvirt,rlaager/libvirt,datto/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,datto/libvirt,eskultety/libvirt,taget/libvirt,datto/libvirt,libvirt/libvirt,eskultety/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,taget/libvirt,andreabolognani/libvirt,olafhering/libvirt,jardasgit/libvirt,zippy2/libvirt,datto/libvirt,crobinso/libvirt,fabianfreyer/libvirt,libvirt/libvirt,jardasgit/libvirt,taget/libvirt,taget/libvirt,fabianfreyer/libvirt,eskultety/libvirt,jfehlig/libvirt