KVM: PPC: Book3S PR: Enable use on POWER9 inside HPT-mode guests

This relaxes the restriction on using PR KVM on POWER9.  The existing
code does work inside a guest partition running in HPT mode, because
hypercalls such as H_ENTER use the old HPTE format, not the new
format used by POWER9, and so no change to PR KVM's HPT manipulation
code is required.  PR KVM will still refuse to run if the kernel is
using radix translation or if it is running bare-metal.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>