When using with QEMU v4.1.0-rc0 or newer, the patch https://libvirt.org/git/?p=libvirt.git;a=commit;h=63acb7bfd56f117309e4fcaf438639d4d7bc7dcb cause some windows hyperv features cannot be enabled.

# virsh start win10

error: Failed to start domain win10

error: unsupported configuration: host doesn't support hyperv 'vpindex' feature

  <features>

    <hyperv>

      <relaxed state='on'/>

      <vapic state='on'/>

      <spinlocks state='on' retries='4096'/>

      <vpindex state='on'/>

      <runtime state='on'/>

      <synic state='on'/>

      <reset state='on'/>

      <frequencies state='on'/>

      <reenlightenment state='on'/>

      <tlbflush state='on'/>

      <ipi state='on'/>

      <evmcs state='on'/>

</hyperv>