feat: igb nic, jumbo frames and tap offloads on the guest link

scripts/libvirt-hook-vnet-offload.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+# libvirt qemu hook: turn GSO and GRO on for the guest's tap.
+#
+# QEMU leaves them off and sets the tap up after udev has run, so it has to
+# happen here. Paired with MTU 9000 they are the difference between 2.5 and
+# 14 Gbit/s into the guest; neither helps alone. Tap names come from the domain
+# XML on stdin, so the hook never calls virsh, which would deadlock libvirtd.
+
+[ "$2" = started ] || exit 0
+
+for tap in $(grep -oE "<target dev='(vnet|tap|macvtap)[^']*'" | sed "s/.*dev='//; s/'$//"); do
+    ethtool -K "$tap" gso on gro on 2>/dev/null
+done
+
+exit 0