golang/xenlight: implement constructor generation

golang/xenlight: implement constructor generation

Generate constructors for generated Go types. Call libxl_<type>_init so
the Go type can be properly initialized.

If a type has a keyed union field, add a parameter to the function
signature to set the key variable, and call the init function for the
keyed union.

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