[review] New 'MMSharedCinterion' interface

[Aleksander Morgado]

Hey,

See this MR:
https://gitlab.freedesktop.org/mobile-broadband/ModemManager/merge_requests/26

It implements a new shared interface containing the code shared
between the Cinterion modem objects, similar to what's been suggested
for Qmi and Xmm in other MRs.

It also fixes the logic behind the parent interface management: we
cannot get a single static pointer pointing to the parent interface
because this will be different depending on the object hierarchy (e.g.
the location interface of the parent of MMBroadbandModemCinterion is
the location interface of the MMBroadbandModem; while the location
interface of the parent of the MMBroadbandModemQmiCinterion is the
location interface of the MMBroadbandModemQmi). This issue would make
ModemManager crash if a MMBroadbandModemQmiCinterion and a
MMBroadbandModemCinterion modem were managed at the same time.

-- 
Aleksander
https://aleksander.es