softmmu/runstate.c: add RunStateTransition support form COLO to PRELAUNCH

If the checkpoint occurs when the guest finishes restarting
but has not started running, the runstate_set() may reject
the transition from COLO to PRELAUNCH with the crash log:

{"timestamp": {"seconds": 1593484591, "microseconds": 26605},\
"event": "RESET", "data": {"guest": true, "reason": "guest-reset"}}
qemu-system-x86_64: invalid runstate transition: 'colo' -> 'prelaunch'

Long-term testing says that it's pretty safe.

Signed-off-by: Like Xu <like.xu@linux.intel.com>
Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

softmmu/runstate.c

@@ -126,6 +126,7 @@ static const RunStateTransition runstate_transitions_def[] = {
     { RUN_STATE_RESTORE_VM, RUN_STATE_PRELAUNCH },
 
     { RUN_STATE_COLO, RUN_STATE_RUNNING },
+    { RUN_STATE_COLO, RUN_STATE_PRELAUNCH },
     { RUN_STATE_COLO, RUN_STATE_SHUTDOWN},
 
     { RUN_STATE_RUNNING, RUN_STATE_DEBUG },