When privateNetwork.addNetwork() fails during xapi connection, the network is
already inserted in privateNetwork.networks and a center may have been set
without being recorded in _starCenters, leaving a broken PrivateNetwork
registered.
After the registration loop, remove the network that failed to register, drop
the orphaned center, and delete the PrivateNetwork once it has no network left,
like _handleDisconnectedXapi already does.
Also defer pushing to noVniNetworks until the network is registered, so a failed
network gets no VNI and does not trigger _electNewCenter outside of any
try/catch.
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