Everything that supports VT-x at all supports nested virtualization -- the outermost hypervisor (called L0 usually) simply emulates the VT-x instructions use by its guest (called L1). The guest's guest is called L2, and L0 switches back and forth between running L1 and L2 as needed.

The problem is that this is extremely slow. Haswell's shadow VMCS makes it less slow, but I doubt it performs particularly well. Another downside is that the CPU only supports two levels of page tables, so it gets awkward to make everything work securely and correctly without emulating a level of page tables as well.