KVM: PPC: Book3S PR: Enable HTM for PR KVM for KVM_CHECK_EXTENSION ioctl

With current patch set, PR KVM now supports HTM. So this patch turns it
on for PR KVM.

Tested with:
https://github.com/justdoitqd/publicFiles/blob/master/test_kvm_htm_cap.c

Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>