KVM: PPC: Make BAT only guest segments work

When a guest sets its SR entry to invalid, we may still find a
corresponding entry in a BAT. So we need to make sure we're not
faulting on invalid SR entries, but instead just claim them to be
BAT resolved.

This resolves breakage experienced when using libogc based guests.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>