ppc: Check the availability of transactional memory

KVM-PR currently does not support transactional memory, and the
implementation in TCG is just a fake. We should not announce TM
support in the ibm,pa-features property when running on such a
system, so disable it by default and only enable it if the KVM
implementation supports it (i.e. recent versions of KVM-HV).
These changes are based on some earlier work from Anton Blanchard
(thanks!).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>