Bearers will ask for data ports that can be used for connections. Right now,
only one data port is expected to be handled, but we don't impose any
restriction in the API.
When the port is grabbed, we setup NULL callbacks for all unsolicited
registration message handlers. If we do enable 3GPP registrations
afterwards, we'll set proper callbacks.
When the first AT port is grabbed, modem initialization is launched, using that
specific port. This port is also passed to all interface initialization methods.
Basically, a replacement of the MMModemBase type, being prepared to handle
multimode devices.
This object derives from a MmGdbusObjectSkeleton, which makes it suitable to be
controlled within the GDBusObjectManagerServer.