KCM: Return a valid tevent error code if a request cannot be created
    
    Previously we were returning whatever was in 'ret' which is wrong,
    typically it would have been EOK as returned from a previous successfull
    call or even an uninitialized value.
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>