Fix the qemu crash when guest shutdown during checkpoint

This patch fixes the following:
    qemu-system-x86_64: invalid runstate transition: 'colo' ->'shutdown'
    Aborted (core dumped)

Signed-off-by: Lei Rao <lei.rao@intel.com>
Reviewed-by: Li Zhijian <lizhijian@fujitsu.com>
Reviewed-by: Zhang Chen <chen.zhang@intel.com>
Reviewed-by: Lukas Straub <lukasstraub2@web.de>
Tested-by: Lukas Straub <lukasstraub2@web.de>
Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>