lib: fix iteration over YANG presence containers

lib: fix iteration over YANG presence containers

State-only and configuration presence-containers need to be treated
differently when iterating over YANG operational data. Currently the
get_elem() callback is used to know when a state-only p-container
exists or not, and configuration p-containers are assumed to always
exist, which is clearly wrong. Fix this by checking the running
configuration to know whether a rw p-container exists or not.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>