libvirt network driver

Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
Add pkg/rpcfd

This is a small unix-domain-sockets only wrapper around net/rpc
that adds support for passing file descriptors in rpc messages.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: jpoimboe)
Add .dockersocket directory to init layer

This will be used by the libvirt exec driver to mount in a directory
with the rpc socket, allowing the daemon to interact with the dockerinit

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: jpoimboe)
pkg/cgroups: Export ParseCgroupFile

This is needed for the libvirt exec driver.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: jpoimboe)
make TestEnv work with libvirt

Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
libvirt execdriver

TODO:
- machine container mode
- cleanup

Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: jpoimboe)