golang/xenlight: implement keyed union C to Go marshaling

golang/xenlight: implement keyed union C to Go marshaling

Switch over union key to determine how to populate 'union' in Go struct.

Since the unions of C types cannot be directly accessed in cgo, use a
typeof trick to typedef a struct in the cgo preamble that is analagous
to each inner struct of a keyed union. For example, to define a struct
for the hvm inner struct of libxl_domain_build_info, do:

  typedef typeof(((struct libxl_domain_build_info *)NULL)->u.hvm) libxl_domain_build_info_type_union_hvm;

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