hw/ppc/spapr: Halt CPU when powering off via RTAS call

hw/ppc/spapr: Halt CPU when powering off via RTAS call

The LoPAPR specification defines the following for the RTAS
power-off call: "On successful operation, does not return".
However, the implementation in QEMU currently returns and runs
the guest CPU again for some more cycles. This caused some
trouble with the new ppc implementation of the kvm-unit-tests
recently. So let's make sure that the QEMU implementation
follows the spec, thus stop the CPU to make sure that the
RTAS call does not return to the guest anymore.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Tested-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>