nodedev: update netdev feature bits before each dumpxml

As with several other attributes of devices (link status, sriov VF
list, IOMMU group list), the detdev feature bits aren't automatically
updated in the nodedev driver's cache when they change. In order to
get a properly up-to-date list when getting the XML of a device, we
must reget them in update-caps prior to each dumpxml.

This resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1232880

src/node_device/node_device_driver.c

@@ -58,6 +58,9 @@ static int update_caps(virNodeDeviceObjPtr dev)
         case VIR_NODE_DEV_CAP_NET:
             if (virNetDevGetLinkInfo(cap->data.net.ifname, &cap->data.net.lnk) < 0)
                 return -1;
+            virBitmapFree(cap->data.net.features);
+            if (virNetDevGetFeatures(cap->data.net.ifname, &cap->data.net.features) < 0)
+                return -1;
             break;
         case VIR_NODE_DEV_CAP_PCI_DEV:
            if (nodeDeviceSysfsGetPCIRelatedDevCaps(dev->def->sysfs_path,