The libvirt slave plugin was built with a very old version of Jenkins, and it can't support current functionality for slaves.  In particular, 'workDir' and 'remoteDir' in the slave configuration are not transferred to the slave during startup (see also https://issues.jenkins-ci.org/browse/JENKINS-47834  The JNLPLauncher class that was used to build the plugin in 2015 doesn't contain the "RemotingWorkDirSettings workDirSettings" field, and the VirtualMachineLauncher class defined in the plugin doesn't support the fields required by "slave-agent.jnlp.jelly".