do not reset no_shutdown after we shutdown the vm

do not reset no_shutdown after we shutdown the vm

Daniel P. Berrange sent a libvirt's patch to support
reboots with the QEMU driver. He implements it in
json model like this:
1. add -no-shutdown in the qemu's option:
   qemu -no-shutdown xxxx
2. shutdown the vm by monitor command system_powerdown
3. wait for shutdown event
4. reset the vm by monitor command system_reset

no_shutdown will be reset to 0 if the vm is powered down.
We only can reboot the vm once.

If no_shutdown is not reset to 0, we can reboot the vm
many times.

Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>