Comment # 7 on bug 1219180 from Fabian Vogt

That explains it. Looking at the EDK II source, the driver for virtio-gpu-pci
does not expose a linear framebuffer, only virtio-vga does
(https://github.com/tianocore/edk2/blob/952b5cf94c8727b65e04d3d507c0134743be2a66/OvmfPkg/Virtio10Dxe/Virtio10.c#L1029).
virtio-vga is not available in qemu-system-aarch64 though.

So a question for the virtualization team (qemu + ovmf): What's the right
controller to use to get this to work? I guess ramfb might work but it's too
limited to be usable in practice.