When you say Chinese businessmen, or Iranian businessmen, it somehow gives the idea they would care more about the country than about their investment. Rather call them businessmen from Iran, or businessmen from China...
If China invests its money in Iranian oil, how much is it going to depend on that to fuel its economy? All the countries around the world keep substitute oil suppliers, because we never know what characteristic some particular country is going to display in future. China's investment in Iran will not be so big as to make China dependent on Iran. It will always remain to the point where Iran gets some money, and China gets some oil, and that's it.
Secondly, there are greater number of businessmen in the US, EU, Saudia etc. with anytime more money and influence than Chinese and Iranian businessmen. It will be more like some financially well off people of just two countries against the influential ones of the whole world. It would not make any business sense to turn others into your enemy, unless you have absolutely no other option left. But the options for Chinese are open, in the EU as well as in the US.
So the question is, what will be China's gain in becoming such a "close friend" of Iran at the cost of making rest of the influential world "angry" with it? Will Iran sell it at a cheaper rate? Or will Iran eat up all the RMB printed by China? Neither.
The character of businessmen and business houses works like Piranhas. The moment you appear weak, all others will eat you up.
Iran will have to come up with an aggressive diplomacy on its part, and learn to be flexible in the current times if it wants to attract big investments in its land to get rid of dependence on natural reserves (that is the main goal of all the countries - to be independent of the natural, but limited, reserves).