ppc/kvm: Register also a generic spapr CPU core family type

There is a regression with the "-cpu" parameter introduced by
the spapr CPU hotplug code: We used to allow to specify a
"CPU family" name with the "-cpu" parameter when running on KVM so
that the user does not need to know the gory details of the exact
CPU version of the host CPU. For example, it was possible to
use "-cpu POWER8" on a POWER8E host CPU. This behavior does not
work anymore with the new hot-pluggable spapr-cpu-core types.
Since libvirt already heavily depends on the old behavior, this
is quite a severe regression in the QEMU parameter interface.
Let's fix it by supporting a CPU family type for the spapr-cpu-core
on KVM, too.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1363812
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
