spapr: Forbid nested KVM-HV in pre-power9 compat mode

spapr: Forbid nested KVM-HV in pre-power9 compat mode

Nested KVM HV only works if the kernel is using the radix MMU mode, ie.
the CPU is POWER9 and it is not running in some pre-power9 compat mode.
Otherwise, the KVM HV module fails to load in the guest with -ENODEV.
It might be painful for a user to discover this late that nested cannot
work with their setup. Erroring out at machine init instead seems to be
the best we can do.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Message-Id: <159491948127.188975.9621435875869177751.stgit@bahia.lan>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>