qemu: lifecycle: make agent-mode shutdown and reboot timeout
    
    When we shutdown/reboot a guest using agent-mode, if the guest itself blocks infinitely,
    libvirt would block in qemuAgentShutdown() forever.
    Thus, we set a timeout for shutdown/reboot, from our experience, 60 seconds would be fine.
    
    Signed-off-by: Zhang Bo <oscar.zhangbo@huawei.com>
    Signed-off-by: Wang Yufei <james.wangyufei@huawei.com>