udev-util: add generic device_get_{vendor,model}_string() helpers

We chck the same props in various places, add a single implementation of
a call to inquire this.

src/core/device.c

@@ -516,9 +516,7 @@ static int device_update_description(Unit *u, sd_device *dev, const char *path)
 
         desc = path;
 
-        if (dev &&
-            (sd_device_get_property_value(dev, "ID_MODEL_FROM_DATABASE", &model) >= 0 ||
-             sd_device_get_property_value(dev, "ID_MODEL", &model) >= 0)) {
+        if (dev && device_get_model_string(dev, &model) >= 0) {
                 desc = model;
 
                 /* Try to concatenate the device model string with a label, if there is one */