The GInterface structure for MMModemIcera isn't instance data, thus we
shouldn't be storing an instance pointer in it. Instead, make implemtors
store the intstance data in their private structure, and have them
implement an accessor for the Icera-private data. This makes everone
(especially GObject) happy. It's a bit of additional indirection, but
we still get to use the MM_MODEM_ICERA_GET_PRIVATE() and we still
get to cast the passed-in GInterface MMModemIcera into the various
GSM MMModem subclasses, which is all we ever wanted anyway.
Implement connect, disconnect, and IP4 config stuff.
Also fix handling of Icera private data. After creation we
need to use MM_MODEM_ICERA_GET_INTERFACE(m)->priv to get the
private data instead of just dereferencing the MMModemIcera,
for reasons that I don't know. If this isn't done, data
gets silently corrupted because writes to the private data
are going into a random location in the object. This a
side-effect of the slightly hack-ish way that MMModemIcera
is a GInterface with private data.
