target/ppc: fix memory leak in kvmppc_is_mem_backend_page_size_ok()

The string returned by object_property_get_str() is dynamically allocated.

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