hw/ppc/spapr: Silence deprecation message in qtest mode

When running "make check", there is currently always an error message
saying "spapr-pci-vfio-host-bridge is deprecated". This happens because
the QOM tests are instantiating all possible devices, and the error
message is currently located in the instance_init() function of the
device. Since it is legal for the tests to instantiate a device without
using it, the error message should be silenced when we're running in
test mode.

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