From ch. 3.1.1 Mac OS Guests of the User Manual:
Mac OS is commercial, licensed software and contains both license and technical restrictions that limit its use to certain hardware and usage scenarios. You must understand and comply with these restrictions.
In particular, Apple prohibits the installation of Mac OS on non-Apple hardware.
These license restrictions are also enforced on a technical level. Mac OS verifies that it is running on Apple hardware. Most DVDs that accompany Apple hardware check for the exact model. These restrictions are not circumvented by Oracle VM VirtualBox and continue to apply.
In the really rare case (0.001%) that you are actually dealing with an Apple computer, in which you have installed Windows/Linux/other, and you're trying to boot a legitimate Mac OS guest and not a Hackintosh, pm a moderator and we'll remedy the situation. Be prepared to provide solid evidence in such a case.