qemu.tests.cpu_hotplug: Make cpu hotplug command configurable.

cpu_set command will be removed in new qemu. So make cpu hotplug
 command configurable.

'cpu_set 1 online' will always sent out as human monitor command no matter
 qmp support cpu_set or not.

Update it to 'cpu_set cpu=1,state=online'. send_args_cmd() will transfer
it to correct human monitor command 'cpu_set 1 online'. Or correct qmp
monitor command if qmp support it.

Signed-off-by: Feng Yang <fyang@redhat.com>