KVM: PPC: rename CONFIG_KVM_E500 -> CONFIG_KVM_E500V2

The CONFIG_KVM_E500 option really indicates that we're running on a V2 machine,
not on a machine of the generic E500 class. So indicate that properly and
change the config name accordingly.

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