storage_backend: Resolve Coverity issue

The switch statement in 'virStorageBackendCreateQemuImgOpts' used the
for loop end condition 'VIR_STORAGE_FILE_FEATURE_LAST' as a possible value,
but since that cannot happen Coverity spits out a DEADCODE message. Adding
the Coverity tag just removes the Coverity message

src/storage/storage_backend.c

@@ -663,6 +663,7 @@ virStorageBackendCreateQemuImgOpts(char **opts,
                     }
                     break;
 
+                /* coverity[dead_error_begin] */
                 case VIR_STORAGE_FILE_FEATURE_LAST:
                     ;
                 }