kerneldevice,generic: load virtual devices without default rules

This is so that "make check" doesn't depend on having the rule files
installed in the default udev rules directory, which currently would
break the build:

    TEST: test-service-generic... (pid=2601)
      /MM/Service/Generic/enable-disable:                                  Activating service name='org.freedesktop.ModemManager1'
    Successfully activated service 'org.freedesktop.ModemManager1'

    ** (/home/aleksander/Development/foss/ModemManager/plugins/.libs/lt-test-service-generic:2601):
    ERROR **: Error setting test profile: GDBus.Error:org.freedesktop.ModemManager1.Error.Core.Failed: Failed to find primary AT port
    FAIL
    GTester: last random seed: R02S3897abaae9df36f8d2eeb679406ab675
    make[3]: *** [Makefile:3804: test-nonrecursive] Terminated

src/mm-plugin.c

@@ -962,7 +962,10 @@ mm_plugin_create_modem (MMPlugin  *self,
             mm_kernel_event_properties_set_subsystem (properties, "virtual");
             mm_kernel_event_properties_set_name (properties, virtual_ports[i]);
 
-            kernel_device = mm_kernel_device_generic_new (properties, &inner_error);
+            /* Give an empty set of rules, because we don't want them to be
+             * loaded from the udev rules path (as there may not be any
+             * installed yet). */
+            kernel_device = mm_kernel_device_generic_new_with_rules (properties, NULL, &inner_error);
             if (!kernel_device) {
                 mm_warn ("Could not grab port (virtual/%s): '%s'",
                          virtual_ports[i],