golang/xenlight: implement keyed union Go to C marshaling

golang/xenlight: implement keyed union Go to C marshaling

Since the C union cannot be directly populated, populate the fields of the
corresponding C struct defined in the cgo preamble, and then copy that
struct as bytes into the byte slice that Go uses as the union.

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