Add a libvirt execdriver

I rebased jpoimboe and alexlarsson work on libvirt execdriver on master and completed it. The original branch is: https://github.com/jpoimboe/docker/tree/libvirt3.

Some points to highlight:
* This execdriver doesn't take advantage of the recent additions in libvirt-lxc for setting the capabilities, so keep-caps / drop-caps isn't supported for the while.
* Some of the code from the lxc driver has been extracted in `execdriver/utils.go` to avoid code duplication.