Build using this command line:

make CONFIG=vbox bin/intel--virtio-net--pcnet32.isarom

Max size of a VirtualBox ROM is 56KB, 57344 bytes. There should be no need
to pad the image as long as the binary is smaller or equal to this size.

To use the ROM in VirtualBox you need to enable it using this command:

vboxmanage setextradata global \
    VBoxInternal/Devices/pcbios/0/Config/LanBootRom \
    /absolute/path/to/intel--virtio-net--pcnet32.isarom

NB: If you build the ROM using the .rom prefix then it'll be built as a PCI
ROM, which won't work properly in VirtualBox.  The error message you'll see
is "No more network devices", which is somewhat confusing.  If you enter the
shell and use the "autoboot" command things will work as intended.  Remember
to always build as a .isarom to avoid this issue.