golang/xenlight: generate enum types from IDL

golang/xenlight: generate enum types from IDL

Introduce gengotypes.py to generate Go code the from IDL. As a first step,
implement 'enum' type generation.

As a result of the newly-generated code, remove the existing, and now
conflicting definitions in xenlight.go. In the case of the Error type,
rename the slice 'errors' to 'libxlErrors' so that it does not conflict
with the standard library package 'errors.' And, negate the values used
in 'libxlErrors' since the generated error values are negative.

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