golang/xenlight: Create stub package

golang/xenlight: Create stub package

Create a basic Makefile to build and install libxenlight Golang
bindings. Also add a stub package which only opens libxl context.

Include a global xenlight.Ctx variable which can be used as the
default context by the entire program if desired.

For now, return simple errors. Proper error handling will be
added in next patch.

Until we get configure support, disable it by default.  It can be
enabled either by adding "CONFIG_GOLANG=y" to .config, or adding it to
the 'make' line.

Signed-off-by: Ronald Rojas <ronladred@gmail.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@citrix.com>