Hi, about using OSX in virtual machines, from EULA of osX Sieraa:
”The grants set forth in this License do not permit you to, and you
agree not to, install, use or run the Apple Software on any non-Apple-branded computer, or to
enable others to do so”
”The grant set forth in Section 2B(iii) above does not permit you to use the virtualized copies or
instances of the Apple Software in connection with service bureau, time-sharing, terminal
sharing or other similar types of services.”
I am not a lawyer, but that also the information I got if we had a OSX project on our last company.
Well, I know doing this for development or testing ist almost common, but I had a quick view inside, for example you need to add an devicekey "ourhardworkbythesewordsguardedpleasedontsteal(c)AppleComputerInc", that means you need to bypass hardware restrictions…
For private “just for fun” yea, you may do - nobody will care about - but for an comercial product … well …. Better to use just on real (expensive) apple hardware.
But in that case you need to have hardware for all old versions… that’s intersting… how to handle that? That’s why I’m sure, virtualization of OSX is common on development and testing 
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Even overalll, the biggest problem would be the testing effort for every new mp3tag version.
Every change could mess up functionallity on old OSX and may need unequal treatment for different OS versions.
regards
Peter