src: Mention DEVICE_REMOVAL_FAILED event in virDomainDetachDeviceAlias docs
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1598087
    
    We are mentioning the positive outcome of the function and not
    the case when live detaching a device is denied and event is
    issued.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Reviewed-by: Jiri Denemark <jdenemar@redhat.com>