only set the cpu model when using a custom cpu mode
only set the cpu model when using a custom cpu mode

according to https://libvirt.org/formatdomain.html#elementsCPU
setting the model is not supported when using "host-model" and
with recent libvirt this actually results in errors like this:
 Call to virDomainCreateWithFlags failed:
  the CPU is incompatible with host CPU:
  Host CPU does not provide required features: svm

vagrant-libvirt/vagrant-libvirt,pradels/vagrant-libvirt,vagrant-libvirt/vagrant-libvirt,infernix/vagrant-libvirt,infernix/vagrant-libvirt,electrofelix/vagrant-libvirt,infernix/vagrant-libvirt,electrofelix/vagrant-libvirt,vagrant-libvirt/vagrant-libvirt,pradels/vagrant-libvirt,electrofelix/vagrant-libvirt