migration: Support zerocopy transmit
Goal
Support Qemu's zero-copy-send migration mode
Technical details
zero-copy-send is a performance enhancement that's about to go into qemu which can speed up migration by reducing CPU usage on the transmit side; it needs switching on by a migration parameter, and thus needs simple libvirt support.
see: https://lists.gnu.org/archive/html/qemu-devel/2022-04/msg04532.html