kvm tools: PPC64, add HPT/SDR1 for -PR KVM

Allocate a page table and point SDR1 to it in order to support the -PR
PPC64 KVM mode.  (The alternative, -HV mode, is available only on a small
set of machines.)

This patch also removes the previous dependency on mapping guest RAM with
huge pages; PR KVM doesn't require them so the user isn't forced to use them.

A new option, '--hugetlbfs default', uses a default path for 16M pages for
HV mode, if required.

Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Pekka Enberg <penberg@kernel.org>