KVM: PPC: Book3S HV: Use the hardware referenced bit for kvm_age_hva

This uses the host view of the hardware R (referenced) bit to speed
up kvm_age_hva() and kvm_test_age_hva().  Instead of removing all
the relevant HPTEs in kvm_age_hva(), we now just reset their R bits
if set.  Also, kvm_test_age_hva() now scans the relevant HPTEs to
see if any of them have R set.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>