kvm/ppc/booke64: Disable e6500 support

The previous patch made 64-bit booke KVM build again, but Altivec
support is still not complete, and we can't prevent the guest from
turning on Altivec (which can corrupt host state until state
save/restore is implemented).  Disable e6500 on KVM until this is
fixed.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>