qemu: fix building autopkgtest image for riscv64
'autopkgtest-buildvm-ubuntu-cloud -v --arch riscv64 --release noble' fails (LP: #2056764).
The QEMU riscv64 virt machine provides ACPI tables by default. ACPI support in the Linux kernel is still work in progress. Hence stick to device-trees.
Use EDK II as firmware to boot via EFI.