staging: vchi: Unify struct shim_service and struct vchi_service_handle

staging: vchi: Unify struct shim_service and struct vchi_service_handle

The idea behind struct vchi_service_handle is to create an opaque handle
to struct shim_service. This can be achieved by doing a forward
declaration of struct shim_service, which will avoid unwarranted casts
and pointer play.

Ultimately as a rename is due all over the vchi user space, rename
struct shim_service into struvt vchi_service, which is more consistent
with the rest of the exposed API.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200629150945.10720-13-nsaenzjulienne@suse.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>