PPC: e500: advertise 4.2 MPIC only if KVM supports EPR

Older KVM versions don't support EPR which breaks guests when we announce
MPIC variants that support EPR.

Catch that case and expose only MPIC version 2.0 which tells the guest that
we don't support the EPR capability yet.

Signed-off-by: Stuart Yoder <stuart.yoder@freescale.com>
[agraf: Add comment, route cap check through kvm_ppc.c]
Signed-off-by: Alexander Graf <agraf@suse.de>
