generator: Add visibility to action struct

The visibility field in action replaces in_fish, in_docs and internal.
The defined types are:

VPublic:
  A public API. This is exported and documented in all language
  bindings, and in guestfish.

VStateTest:
  A public API which queries the library state machine. It is exported
  and documented in all language bindings, but not guestfish.

VBindTest:
  An internal API used only for testing language bindings. It is
  guarded by GUESTFS_PRIVATE in the C api, but exported by all other
  language bindings as it is required for testing. If language
  bindings offer any way to guard use of these apis, that mechanism
  should be used. It is not documented anywhere.

VDebug:
  A debugging API. It is exported by all language bindings, and in
  guestfish, but is not documented anywhere.

VInternal:
  An internal-only API. It is guarded by GUESTFS_PRIVATE in the C api,
  and not exported at all in any other language binding. It is not
  documented anywhere.