KVM: PPC: make e500v2 kvm and e500mc cpu mutually exclusive

We can't run e500v2 kvm on e500mc kernels, so indicate that by
making the 2 options mutually exclusive in kconfig.

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