It is not possible to set in xWMS configuration file that some layers are not shown by default. Now it appears that it attaches all listed layers from WMS at once. For example if there are multiple orthophotos of same area but with different dates then it would be more easier to switch layers than to attach multiple xWMS files for each layer. Tried to delete layers from key or key but result is not as expected.
There is already a change request from year 2010 but most likely lost - CR (#306124).