network: change cleanup: to success/cleanup/error: in network*() functions

network: change cleanup: to success/cleanup/error: in network*() functions

A later patch will be adding a counter that will be
incremented/decremented each time an guest interface starts/stops
using a particular network. For this to work, all types of networks
need to go through a common return sequence rather than returning
early. To setup for this, a new success: label is added (when
necessary), a new error: label is added which does any cleanup
necessary only for error returns and then does goto cleanup, and early
returns are changed to goto error if it's a failure, or goto success
if it's successful. This way the intent of all the gotos is
unambiguous, and a successful return path never encounters the
"error:" label.