Pass modules through to the guest, if applicable

This only works with --installed-kernel for now.  It is likely to
cause problems guest kernels that are missing this fix:

    commit b6f52ae2f0d32387bde2b89883e3b64d88b9bfe8
    Author: Richard Yao <ryao@gentoo.org>
    Date:   Sat Feb 8 19:32:01 2014 -0500

        9p/trans_virtio.c: Fix broken zero-copy on vmalloc() buffers

Signed-off-by: Andy Lutomirski <luto@amacapital.net>