target-ppc: Remove hack for ppc_hash64_load_hpte*() with HV KVM

With HV KVM, the guest's hash page table (HPT) is managed by the kernel and
not directly accessible to QEMU.  This means that spapr->htab is NULL
and normally env->external_htab would also be NULL for each cpu.

However, that would cause ppc_hash64_load_hpte*() to do the wrong thing in
the few cases where QEMU does need to load entries from the in-kernel HPT.
Specifically, seeing external_htab is NULL, they would look for an HPT
within the guest's address space instead.

To stop that we have an ugly hack in the pseries machine type code to
set external htab to (void *)1 instead.

This patch removes that hack by having ppc_hash64_load_hpte*() explicitly
check kvmppc_kern_htab instead, which makes more sense.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
