libvirt: create libvirt-lxc plugins

Create container and network plugins for libvirt-lxc.  If libvirtd is
running, use the libvirt plugins.  Otherwise use the dockernet and lxc
plugins.

With this change, docker must be dynamically linked since libvirt
doesn't seem to support being statically linked, based on the compile
errors seen.
workaround for libvirt /.oldroot directory leak
workaround for upstream libvirt bug (error on > 60 chars)

This is a temporary fix for
https://bugzilla.redhat.com/show_bug.cgi?id=1033369
dind: fix cpuacct,cpu workaround for libvirt

When running the dind script in a libvirt container, it shows the
following error:

  ln: failed to create symbolic link `/sys/fs/cgroup/cpu,cpuacct/cpuacct,cpu': Operation not permitted

The libvirt-lxc already creates the cpu,cpuacct cgroup directory, so
check for its existence before trying the symlink.