Add support for checkpoint and restore via CRIU

This patch adds support for checkpointing and restoring containers via CRIU.
It adds two api calls, ->checkpoint and ->restore, which are wrappers around
the CRIU CLI. CRIU has an RPC API, but reasons for preferring exec() are
discussed in [1].

To checkpoint, users specify a directory to dump the container metadata (CRIU
dump files, plus some additional information about veth pairs and which
bridges they are attached to) into this directory. On restore, this
information is read out of the directory, a CRIU command line is constructed,
and CRIU is exec()d. CRIU uses the lxc-restore-net callback (which in turn
inspects the image directory with the NIC data) to properly restore the
network.

This will only work with the current git master of CRIU; anything as of
a152c843 should work. There is a known bug where containers which have been
restored cannot be checkpointed [2].

[1]: http://lists.openvz.org/pipermail/criu/2014-July/015117.html
[2]: http://lists.openvz.org/pipermail/criu/2014-August/015876.html

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>