I have a container that runs a 4.3 BSD userland using opensimh; it's not super hard to set up, just takes a bit of patience and willingness to learn how opensimh works.
Nested virtualization for certain x86 OSes running in QEMU is supported, although you will have to enable it manually (VirtualBox has a checkbox for this in its settings). For VMs that support it, the QEMU launch scripts will automatically use KVM if available and fall back to TCG if nested virtualization isn't enabled.