librados: hide RadosWhereis

The librados symbols are now cleanly exported explicitly, as opposed to
making all symbols visible. As a consequence the whereis command exposed
by librados.hpp must comply: the dump command must be in librados.hpp
The rados.cc function cannot use the RadosWhereis class directly anymore.

Signed-off-by: Loic Dachary <ldachary@redhat.com>