[pvr.tvh] Added new PVR API function getBackendHostname.

Added new PVR API function getBackendHostname. 

Please refer to https://github.com/opdenkamp/xbmc-pvr-addons/pull/372  and https://github.com/xbmc/xbmc/pull/4342 for details

You "need" to lock `g_mutex` when accessing the global variables.