Build and boot vm.raw with edk2, u-boot. `emulators/qemu31` does not include
edk2, but can be booted with `sysutils/u-boot-qemu-arm64`.
An image generated with this change is available here: https://people.freebsd.org/~mhorne/FreeBSD-13.0-CURRENT-arm64-aarch64-gpt.raw.zstd
Launch with built-in EDK2:
```
$ qemu-system-aarch64 -cpu cortex-a57 -machine virt -m 4G -nographic -drive if=none,file=FreeBSD-13.0-CURRENT-arm64-aarch64-gpt.raw,format=raw,id=hd0 -device virtio-blk-device,drive=hd0 -bios edk2-aarch64-code.fd
```
Launch with u-boot:
```
$ qemu-system-aarch64 -cpu cortex-a57 -machine virt -m 4G -nographic -drive if=none,file=FreeBSD-13.0-CURRENT-arm64-aarch64-gpt.raw,format=raw,id=hd0 -device virtio-blk-device,drive=hd0 -bios /usr/local/share/u-boot/u-boot-qemu-arm64/u-boot.bin
```