KVM: powerpc: Show timing option only on embedded

Embedded PowerPC KVM has an exit timing implementation to track and evaluate
how much time was spent in which exit path.

For Book3S, we don't implement it. So let's not expose it as a config option
either.

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