bug fix: guest iface does not return model if no model is found

On some distros, libvirt will not set model automatically.
if no model is found,  guest iface will not return this field.

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>
Signed-off-by: Aline Manera <alinefm@br.ibm.com>