Qemu 8.0 and libvirt 9.3.0 added support for qemu emulated igb
network device. This patch adds the new igb value for hw_vif_model
so nova could eventually support booting VMs with such devices.
Subsequent patches will enable libvirt support.
Implements: blueprint igb-vif-model
Change-Id: I9c8dc1a663d0534d62798c5b4c8d4539551f7ae4