Convert vg_list rpc to new style result

This doesn't have known failure modes but converting will help later.

We also now call directly utils.ListVolumeGroups() instead of the
backend.ListVolumeGroups() so that we don't have to undo the (status,
value) result type.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>