Solution:
- Add check for the [count] parameter in zmq_sendiov() and zmq_recviov()
- Use and add test for zmq_sendiov() in tests/test_iov.cpp
- Add error state tests for zmq_sendiov() in tests/test_iov.cpp
- Add error state tests for zmq_recviov() in tests/test_iov.cpp
- Cleanup tests/test_iov.cpp for style, consistency and clarity
- Generally improve test coverage for both API methods

Hat-tip:
@somdoron, @bluca