[virtio] Replace the virtio core and network device driver

The existing virtio network driver has been somewhat hacked together
over the past two decades by multiple contributors, and includes a
substantial amount of logic that is almost but not quite duplicated
between the "legacy" and "modern" code paths.

Rip out the existing driver and replace with a completely new driver
written based on the Virtual I/O Device specification document, not
derived from the Linux kernel driver.

Signed-off-by: Michael Brown <mcb30@ipxe.org>