launch: libvirt: Use host-passthrough instead of host-model.

<cpu model="host-passthrough"> really passes -cpu host to qemu, which
is what we want since we don't care about live migration or ABI
stability.

This should avoid http://bugzilla.redhat.com/870071 .

Note this "taints" the libvirt domain.  We don't particularly care
about that, and the reason for the tainting doesn't seem to make much
sense anyway.

This updates commit 6f76fdb41eb6bd124fbc3d084f5c2a3371b37d9b.