[FIXED JENKINS-16259] use @DataBoundConstructor instead of newInstance()

fixes JENKINS-16259: use @DataBoundConstructor instead of newInstance()

DataBoundConstructor is the recommended way, where newInstacnce() is deprecated. Changing this, allows to use the xunit publisher in the flexible publish plugin: https://wiki.jenkins-ci.org/display/JENKINS/Flexible+Publish+Plugin