golang/xenlight: do not hard code libxl dir in gengotypes.py

golang/xenlight: do not hard code libxl dir in gengotypes.py

Currently, in order to 'import idl' in gengotypes.py, we derive the path
of the libxl source directory from the XEN_ROOT environment variable, and
append that to sys.path so python can see idl.py. Since the the recent move of
libxl to tools/libs/light, this hard coding breaks the build.

Instead, check for the environment variable LIBXL_SRC_DIR, but move this
check to a try-except block (with empty except). This simply makes the
real error more visible, and does not strictly require that
LIBXL_SRC_DIR is used. Finally, update the Makefile to set LIBXL_SRC_DIR
rather than XEN_ROOT when calling gengotypes.py.

Signed-off-by: Nick Rosbrook <rosbrookn@ainfosec.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>