The virtual loopback IP does not work on Windows 2012 R2 server with XenDesktop 7.8.
Loopback will not work if you are using Windows Server 2012 and 2012 R2.
In Windows Server 2012 there are now separate settings for external IP virtualization and loopback IP virtualization, so binding to 127.0.0.1 will always use session 0 unless loopback IP virtualization is configured.