Fix the qemu crash when guest shutdown in COLO mode

Fix the qemu crash when guest shutdown in COLO mode

In COLO mode, if the startup parameters of QEMU include "no-shutdown",
QEMU will crash when the guest shutdown. The root cause is when the
guest shutdown, the state of VM will switch COLO to SHUTDOWN. When do
checkpoint again, the state will be changed to COLO. But the state
switch is undefined in runstate_transitions_def, we should add it.
This patch fixes the following:
qemu-system-x86_64: invalid runstate transition: 'shutdown' -> 'colo'
Aborted

Signed-off-by: Lei Rao <lei.rao@intel.com>
Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Reviewed-by: Zhang Chen <chen.zhang@intel.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>