golang/xenlight: define Defbool builtin type

golang/xenlight: define Defbool builtin type

Define Defbool as struct analagous to the C type, and define the type
'defboolVal' that represent true, false, and default defbool values.

Implement Set, Unset, SetIfDefault, IsDefault, Val, and String functions
on Defbool so that the type can be used in Go analagously to how its
used in C.

Finally, implement fromC and toC functions.

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