Add xenlight and xentoollog

The xenlight module currently depends on changes in libxl that are not
yet upstreamed (but submitted upstream). Therefore, you need a special
version of the Xen RPM to compile the C bindings. The special version
has OCAML_READY #defined in libxl.h, so we are compiling the bindings
conditionally.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Merge pull request #18 from robhoes/master

Add xenlight and xentoollog modules