Build using this command line:

make CONFIG=vbox EMBED=config/vbox/embedded.ipxe bin/intel--virtio-net--pcnet32.rom

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.

The embedded script is required because VirtualBox uses the ROM as an ISA
ROM, which will not perform any autoboot behavior.  The bundled embedded
script reproduces the default autoboot behavior.

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

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