From what I've gathered (and experienced last time I've tried it), Windows support is nonexistent right now. That might change eventually, but OpenBSD and Linux (and I think NetBSD?) are the initial targets.
QEMU is available in packages/ports, though; while almost certainly slow, it's a start. VirtualBox on Linux requires a kernel module, so unless someone manages to port that to OpenBSD (which would translate to adding it to OpenBSD's kernel, which doesn't support loadable kernel modules anymore), that one probably ain't an option.
I cannot comment on whether it is suitable for running VMs at this time.