tests/postcopy: Use KVM on ppc64 only if it is KVM-HV

The ppc64 postcopy test does not work with KVM-PR, and it is also
causing annoying warning messages when run on a x86 host. So let's
use KVM here only if we know that we're running with KVM-HV (which
automatically also means that we're running on a ppc64 host), and
fall back to TCG otherwise.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>