golang/xenlight: re-name Bitmap marshaling functions

golang/xenlight: re-name Bitmap marshaling functions

Re-name and modify signature of toGo function to fromC. The reason for
using 'fromC' rather than 'toGo' is that it is not a good idea to define
methods on the C types. Also, add error return type to Bitmap's toC function.

Finally, as code-cleanup, re-organize the Bitmap type's comments as per
Go conventions.

Signed-off-by: Nick Rosbrook <rosbrookn@ainfosec.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
--
Changes in v2:
- Use consistent variable naming for slice created from
  libxl_bitmap.