util: use netlink to delete bridge devices

https://bugzilla.redhat.com/show_bug.cgi?id=1125755

reported that a stray bridge device was left on the system when a
libvirt network failed to start due to an illegal iptables rule caused
by bad config. Apparently the reason this was happening was that
NetworkManager was noticing immediately when the bridge device was
created and automatically setting it IFF_UP. libvirt would then try to
setup the iptables rules, get an error back, and since libvirt had
never IFF_UPed the bridge, it didn't expect that it needed to set it
~IFF_UP before deleting it during the cleanup process. But the
ioctl(SIOCBRDELBR) ioctl will fail to delete a bridge if it is IFF_UP.

Since that bug was reported, NetworkManager has gotten a bit more
polite in this respect, but just in case something similar happens in
the future, this patch switches to using the netlink RTM_DELLINK
message to delete the bridge - unlike SIOCBRDELBR, it will delete the
requested bridge no matter what the setting of IFF_UP.

src/util/virnetdevbridge.c

 /*
- * Copyright (C) 2007-2014 Red Hat, Inc.
+ * Copyright (C) 2007-2015 Red Hat, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -457,7 +457,15 @@ int virNetDevBridgeCreate(const char *brname)
  *
  * Returns 0 in case of success or an errno code in case of failure.
  */
-#if defined(HAVE_STRUCT_IFREQ) && defined(SIOCBRDELBR)
+#if defined(__linux__) && defined(HAVE_LIBNL)
+int virNetDevBridgeDelete(const char *brname)
+{
+    /* If netlink is available, use it, as it is successful at
+     * deleting a bridge even if it is currently IFF_UP.
+     */
+    return virNetlinkDelLink(brname);
+}
+#elif defined(HAVE_STRUCT_IFREQ) && defined(SIOCBRDELBR)
 int virNetDevBridgeDelete(const char *brname)
 {
     int fd = -1;