qemu.tests:add hotplug_vcpu case

hotplug 159 vcpus and check vcpu number inside and reboot guest

Changelog from V3:
1) removed open/close session code.
2) use vm.monitor.send_args_cmd to auto choose monitor
3) use utils_misc.wait_for() to replace time.sleep()
Changelog from V2:
1) add all error.context with logging.info
Changelog from V1:
1) fixed the hardcode log
2) make the error.context with 'logging.info' argument
3) update the docstring
4) add try / except to the hotplug code
5) sync the log with test steps
6) added 10 Secs sleep time as Win guest needs it

Signed-off-by: Xiaoqing Wei <xwei@redhat.com>
Acked-by: Feng Yang <fyang@redhat.com>
qemu.tests: add hotplug_vcpu.nonexist_vcpu case

Hotplug one nonexistent cpus number

Signed-off-by: Xiaoqing Wei <xwei@redhat.com>