Comment 2 for bug 1713490

The dev is as expected the virtio network card
    <interface type='network'>
      <mac address='52:54:00:cc:a3:b7'/>
      <source network='default'/>
      <model type='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>

There is no explicit rom specified on the guest commandlines, so they pick their defaults.
Trying to list what they load:
 $ strace -e open /usr/bin/qemu-system-x86_64 -M pc-i440fx-trusty -device virtio-net -S -nographic -nodefaults 2>&1 | grep virtio
Shows /usr/lib/ipxe/qemu/efi-virtio.rom in both cases.

And this is actually matching the error sizes 0x4000 = 256k vs 0x8000 = 512k (next best power of 2).

I remember vague to have seen a similar issue long ago, trusty maybe even precise.
After a while I found it - a revival of one of my most hated issues bug 1291321.

Almost tempted to downgrade ipxe in artful to do this properly and with some time into 18.04 - but for now take some time to debug further.