api,introspection: report list of drivers, not just one

Different ports of the same modem may get handled by different drivers. We
therefore need to provide a list of drivers (new `Modem.Drivers' property with
signature 'as') instead of just one (removed `Modem.Driver' property with
signature 's').

$ sudo mmcli -m 0 | grep drivers
           |        drivers: 'qcserial, qmi_wwan'

src/mm-plugin.h

@@ -84,7 +84,7 @@ struct _MMPluginClass {
      * a list of port probes (Mandatory) */
     MMBaseModem *(*create_modem) (MMPlugin *plugin,
                                   const gchar *sysfs_path,
-                                  const gchar *driver,
+                                  const gchar **drivers,
                                   guint16 vendor,
                                   guint16 product,
                                   GList *probes,