plugin-manager: handle race condition during probing

The specific case:
 * Modem exposes cdc-wdm port, Generic plugin probes it successfully as QMI.
 * Modem exposes new ports, including the wwan one. All ttys fail probing
   because they're neither AT nor QCDM (CnS in this case).
 * The wwan port ends up without a port being suggested and is not grabbed.

The root cause of this is that we do not propagate the suggested plugin to newly
added ports when it's the Generic one. If it wasn't the Generic one, the newly
added ports would start with the suggested one for probing.

Now, handle this by looking for the device-specified plugin when the port
probing ends without a specific port given. If there is such a device-specified
plugin accept the port, and otherwise, reject it.

src/mm-plugin-manager.c

@@ -142,8 +142,21 @@ static void
 port_probe_context_finished (PortProbeContext *port_probe_ctx)
 {
     FindDeviceSupportContext *ctx = port_probe_ctx->parent_ctx;
+    MMPlugin *device_plugin;
+
+    /* Get info about the currently scheduled plugin in the device */
+    device_plugin = (MMPlugin *)mm_device_peek_plugin (ctx->device);
 
     if (!port_probe_ctx->best_plugin) {
+        /* If the port appeared after an already probed port, which decided that
+         * the Generic plugin was the best one (which is by default not initially
+         * suggested), we'll end up arriving here. Don't ignore it, it may well
+         * be a wwan port that we do need to grab. */
+        if (device_plugin) {
+            mm_dbg ("(Plugin Manager) [%s] assuming port can be handled by the '%s' plugin",
+                    g_udev_device_get_name (port_probe_ctx->port),
+                    mm_plugin_get_name (device_plugin));
+        } else {
             gboolean cancel_remaining;
             GList *l;
 
@@ -170,12 +183,8 @@ port_probe_context_finished (PortProbeContext *port_probe_ctx)
             if (cancel_remaining)
                 /* Set a NULL suggested plugin, will cancel the probes */
                 suggest_port_probe_result (ctx, port_probe_ctx, NULL);
-
+        }
     } else {
-        MMPlugin *device_plugin;
-
-        device_plugin = (MMPlugin *)mm_device_peek_plugin (ctx->device);
-
         /* Notify the plugin to the device, if this is the first port probing
          * result we got.
          * Also, if the previously suggested plugin was the GENERIC one and now