golang/xenlight: revise use of Context type

golang/xenlight: revise use of Context type

Remove the exported global context variable, 'Ctx.' Generally, it is
better to not export global variables for use through a Go package.
However, there are some exceptions that can be found in the standard
library.

Add a NewContext function instead, and remove the Open, IsOpen, and
CheckOpen functions as a result.

Also, comment-out an ineffectual assignment to 'err' inside the function
Context.CpupoolInfo so that compilation does not fail.

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