When running HVM guests, virtual extensions are enabled in hardware because
Xen is using them.  As a result, a guest can blindly execute the
virtualisation instructions, and will exit to Xen for processing.

In the case that the guest hasn't followed the correct (virtual) configuration
procedure, it shouldn't be able to use the instructions, and Xen should
respond with #UD exception.  When nested virtualisation is disabled for the
guest, it is not permitted to complete the configuration procedure.

Unfortunately, when nested virtualisation is intended to be disabled for the
guest, an incorrect default value leads Xen to believe that the configuration
procedure has already been completed.

References:

https://seclists.org/oss-sec/2018/q4/98