stable/for-linus-4.0-rc1-tag

- Fix two regression introduced in 4.0-rc1 affecting PV/PVH guests in
  certain configurations.
- Prevent pvscsi frontends bypassing backend checks.
- Allow privcmd hypercalls to be preempted even on kernel with
  voluntary preemption.  This fixes soft-lockups with long running
  toolstack hypercalls (e.g., when creating/destroying large domains).