lguest: Convert net device to virtio 1.0 PCI.

The only real change here (other than using the PCI bus) is that we
didn't negotiate VIRTIO_NET_F_MRG_RXBUF before, so the format of the
packet header changed with virtio 1.0; we need TUNSETVNETHDRSZ on the
tun fd to tell it about the extra two bytes.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>