[mythtv-cmyth] Remove PVRClientMythTV::GetArtWork
[cmyth] Swap parameters in cmyth_storagegroup_get_fileinfo

Swap hostname and storagegroup parameters in cmyth_storagegroup_get_fileinfo
to align them with the call to QUERY_SG_FILEQUERY.
[mythtv-cmyth] Add hostname parameter to MythConnection::GetStorageGroupFile
[mythtv-cmyth] Cache backendname in FileOps for calls to GetStorageGroupFile