libxl/remus: setup and control network output buffering

libxl/remus: setup and control network output buffering

This patch adds the machinery required for protecting a guest's
network device state. This patch comprises of two parts:

1. Hotplug scripts: The remus-netbuf-setup script is responsible for
  setting up and tearing down the necessary infrastructure required for
  network output buffering.  This script should be invoked by libxl for
  each of the guest's network interfaces, when starting or stopping Remus.

  Apart from returning success/failure indication via the usual hotplug
  entries in xenstore, this script also writes to xenstore, the name of
  the REMUS_IFB device to be used to control the vif's network output.

  The script relies on libnl3 command line utilities to perform various
  setup/teardown functions. The script is confined to Linux platforms only
  since NetBSD does not seem to have libnl3.

2. Remus network device: Implements the interfaces required by the
   remus abstract device layer. A note about the implementation:

   a) init_subkind_nic() & cleanup_subkind_nic() are called once per Remus
      invocation. They establish and free netlink related state respectively.

   b) setup() and teardown are called for each vif attached to the
      guest.
      During setup():
      i) The hotplug script is called to setup a network buffer on a
         given vif. The script chooses an available IFB device from
         the system, redirects vif egress traffic to the IFB device
         and sets up the plug qdisc (output buffer) on the IFB device.
         The name of the IFB device is communicated via xenstore to
         libxl.

      ii) Libxl obtains a handle to the plug qdisc using the libnl3 API
          and subsequently controls output buffering using this handle
          in the checkpoint callbacks.

      During teardown(), the hotplug scripts are called again to remove
      the vif->ifb traffic redirection, release the ifb and the plug
      qdisc associated with it.

   c) The checkpoint callbacks [postsuspend(), preresume() and commit()]
      are implemented as synchronous ops as the netlink calls associated
      with the qdisc subsystem are very fast.

Signed-off-by: Shriram Rajagopalan <rshriram@cs.ubc.ca>
Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>