Running old-time OS in a virtual machine brings several problems, such as low computer architecture compatibility and bad user experience. Architecture compatibility is good with dedicated emulators, but they do not provide snapshots and other useful features of such mainstream virtualization systems as QEMU. So nested virtualization combining QEMU and some dedicated emulator is helpful in this case. When ancient hardware is supported by QEMU natively, you still have to solve the problem of mouse integration, also known as the problem of noncoinciding cursors.