tests/cpu-plug-test: Check the CPU hot-plugging with device_add, too

Using 'device_add' instead of 'cpu-add' is the new way for
hot-plugging CPUs, so we should test this regularly, too.

Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
Tested-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
