Before this, we only exported the modem to DBus when all ports were organized,
in order to make sure that we select as primary port the one we really want and
not the first AT port grabbed. Given that to get all the ports organized we also
needed to wait to get all the ports grabbed, we can now also defer the creation
of the modem object until all the ports get grabbed. This allows us to create
different types of objects based on the ports available (e.g. we can now create
QMI-supported modem objects if we see a QMI port around).
Lets operations specify that they cannot be run currently, but that the caller
may retry later. Of course, it depends on the caller to really honour the
retry-later suggestion.
To be returned when we try to send an AT command to a modem, but there is no
usable AT port available, as the primary port is connected (hence the name)
and there is no secondary port.
