Summary: important system binaries (such as Finder.app) are encrypted with a key held in the SMC. The OS transparently decrypts them, but needs access to that key to do so.

It is possible to convince QEMU to run OS X: http://www.contrib.andrew.cmu.edu/~somlo/OSXKVM/ (though I've not tried this myself). They do require you to provide the relevant key yourself, I imagine for legal reasons.