librbd: Only public API symbols from the shared library

The librbd shared library was previously exporting all
symbols.  librbd public API methods are now explicitly
exported and all other symbols are hidden.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
@liewegas _LOCAL was leftover from a test to see if I needed to flag private class methods, but current GCC handles the case correctly via the default visibility option.  

Updates pushed.