Allow customizing qemu-kvm and libvirt.

To use customized qemu-kvm and libvirt, please:

1. clone litevirt-repo from /root
$ su -
$ cd /root
$ git clone https://github.com/luohao-brian/litevirt-repo

2. uncomment the following line from litevirt-livecd/spins/f18/litevirt-livecd.ks