Fix unzip invocation in integration tests, and add igb driver

Use unzip -o rather than unzip -u when extracting the kernel to use with stboot; -u (update) leaves any existing and stale file unchanged, and we probably have no reliable timestamps needed for a working update operation.

Add the igb.ko driver and its dependencies to the initramfs qemu-iso-boot-from-net.sh script. That makes the resulting image boot on our supermicro test machine.