Alwas set model = 'netfront' for xen over libvirt.

This will prevent emulated network driver to be attached to virtual machine.
Emulated network driver is not working for xen over libvirt as tap interface
name is suffixed by xen with '-emu' which makes this interface name to long.

Additionaly security groups will not also work for this interface as openstack
is setting up filters on interface without -emu suffix.

Fixes: bug #1202199