Use GVariant for sr_config_*() functions

Use GVariant for sr_config_*() functions

sr_config_get() provides a GVariant owned by the caller, so it must be
released with g_variant_unref() when done.

sr_config_set() takes a GVariant from the caller which may be floating;
it will be properly sunk and release after use by this function. Thus
the output of g_variant_new_*() may be used as an argument.

sr_config_list() also provides a GVariant owned by the caller, to be
unreferenced when done.

sr_config_make() can take a floating reference.