Some host implementations (notably Google Compute Platform) are known
to unconditionally write back VIRTIO_NET_HDR_F_DATA_VALID to
header->flags for received packets, regardless of the features
negotiated by the driver.  This breaks the transmit datapath by
effectively setting an illegal flag for all subsequent transmitted
packets.

Work around this problem by using separate empty header buffers for
the receive and transmit queues.

Debugged-by: Ladi Prosek <lprosek@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>