I've discovered the following additional issues with ESXi on kvm:
1. At present, kvm does not appear to support XSAVE, AVX, AVX2, or BMI2. On my (now fairly ancient) Haswell system, I find I need to invoke qemu-system-x86_64 with "-cpu host,-xsave,-avx,-avx2,-bmi2"
2. There appear to be some issues regarding the communication of CPU topology. For an 8-vCPU ESXi VM, I find that I need to invoke qemu-system-x86_64 with "-smp cores=8,sockets=1"
3. When running under kvm, ESXi 6.0 cannot run VMs with debugging enabled, because it tries to verify the value of a VMCS field that kvm does not implement.
This configuration is still not very stable, but some things do work.