Add hot_add_cpu hook to QEMUMachine

Hook should be set by machines that implement CPU hot-add
via cpu-add QMP command.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>