In the vcpu hotplug code if exit from the monitor failed we would still
attempt to save the status XML. When the daemon is terminated the
monitor socket is closed. In such case, the written status XML would not
contain the monitor path and thus be invalid.

Avoid this issue by only saving status XML on success of the monitor
command.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1439452