Initial version of libvirt execdriver

The backend is only built for the dynamic binary, because of problems
linking statically to libvirt.

TODO:
- Restore() callback
- machine container mode
- test suite

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)