KVM: PPC: Book3S HV: Add handler for HV facility unavailable

At present this should never happen, since the host kernel sets
HFSCR to allow access to all facilities.  It's better to be prepared
to handle it cleanly if it does ever happen, though.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>