The rbd_list method has a quite unpleasant signature returning an
array of strings in a single buffer instead of an array. It is
being deprecated in favour of rbd_list2. To maintain clarity of
code when supporting both APIs in parallel, split the rbd_list
code out into a separate method.

In splitting this we now honour the rbd_list failures.
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>