libvirt-tck was failing several domain tests [1] with qemu 0.14, which
is now less tolerable of specifying 2 bootroms with the same boot index [2].

Drop the 'boot=on' param if kernel has been specfied.

[1] https://www.redhat.com/archives/libvir-list/2011-February/msg00559.html
[2] http://lists.nongnu.org/archive/html/qemu-devel/2011-02/msg01892.html