Fix issue #63: use lxc-execute instead of lxc-start

Here's a first attempt at fixing issue #63. It probably needs additional testing -- I've only tested it with Ubuntu 14.04.1 as the host, and with LXC_SUITE=precise,LXC_ARCH=amd64 and LXC_SUITE=precise,LXC_ARCH=i386 as two guests (I ran through a single gbuild for each and checked that the outputs matched a known good one).