- Rework the XSAVE/XRSTOR emulation to only expose XCR0 features to the
  guest for which the rules regarding xsetbv emulation are known.  In
  particular future extensions like AVX-512 have interdependencies among
  feature bits that could allow a guest to trigger a GP# in the host with
  the current approach of allowing anything the host supports.
- Add proper checking of Intel MPX and AVX-512 XSAVE features in the
  xsetbv emulation and allow these features to be exposed to the guest if
  they are enabled in the host.
- Expose a subset of known-safe features from leaf 0 of the structured
  extended features to guests if they are supported on the host including
  RDFSBASE/RDGSBASE, BMI1/2, AVX2, AVX-512, HLE, ERMS, and RTM.  Aside
  from AVX-512, these features are all new instructions available for use
  in ring 3 with no additional hypervisor changes needed.

Reviewed by:	neel