fix: Slave should be shut down only if idle 

The latest version of the plugin shutdowns a slave, no matter if a job is running or not, if Jenkins master looses the network connection with the slave. This should not happen because Jenkins slaves are capable of reestablishing it. 

The root of the problem is that the slave shutdown happens in VirtulMachineLauncher.afterDisconnect(), triggered everytime when the connection between master and slave goes down. The provided patch fixes that.