tests/cpu-plug-test: Check CPU hot-plugging on ppc64, too

Hot plugging on ppc64 is possible via "device_add", too. Unlike x86,
we must not specify a 'socket-id' and 'thread-id' here, so this needs
to be done with a separate function that just specifies the 'core-id'
during the "device_add".

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