pseries: Under kvm use guest cpu = host cpu by default

pseries: Under kvm use guest cpu = host cpu by default

Now that we've implemented -cpu host for ppc, this patch updates the
pseries machine to use the host cpu as the guest cpu by default when
running under KVM.  This is important because under KVM Book3S-HV the guest
cpu _cannot_ be of a different type to the host cpu (at the moment
KVM Book3S-HV will silently virtualize the host cpu instead of whatever was
requested, but in future it is likely to simply refuse to run the VM if
a cpu model other than the host's is requested).

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>