target-ppc/kvm: Enable transactional memory on POWER8 with KVM-HV, too

Transactional memory is also supported on POWER8 KVM-HV if the
KVM_CAP_PPC_HTM is not available in the kernel yet, so add a hack
to allow TM here, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
