For QCDM devices we want most of what MMSerialPort does, but not
the AT command handling stuff since the commands and responses
aren't AT commands nor are they even strings. So convert everything
that MMSerialPort does into a GByteArray, and let MMAtSerialPort
handle the conversion to strings when necessary.
ZTE modems need to use udev rules to assign port type hints,
so generalize that and port all the plugins over to suggested
port types in the MMModem interface's grab_port() function.
Allow plugins to perform asynchronous port detection, and to defer port detection
until later. This moves the prober bits into MMPluginBase so that all plugins
can take adavantage of it only when needed; the probing is not done at udev time.
Furthermore, plugins like Novatel can flip the secondary ports over the AT mode
through deferred detection, by deferring the secondary ports until the main port
has been detected and AT$NWDMAT has been sent.
This commit also finishes the port of the rest of the plugins (except mbm) over
to the new port detection methods and plugin API.
