Updated - Using the MSbuild Runner, we see the following error
We have set the property in sonar.project.properties sonar.cs.fxcop.fxCopCmdPath=C:\Program Files (x86)\Microsoft Visual Studio 12.0\Team Tools\Static Analysis Tools\FxCop\FxCopCmd.exe
C# Plugin - 4.1 MsBuild Runner - 1.0 SonarQube - 5.1.1