lib.sh: Use host-passthrough when spawning VMs

virsh defaults to using "host-model" instead of "host-passthrough",
which not only might lead to a small performance hit on x86, it is
also causing compatibility issues with libvirt 3.x.

Address this by explicitly requesting "host-passthrough" as the
CPU model for virt-install.

Change-Id: Ia7c5e8ff906c02fed2193c268cbb3594c5607bc6
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>