tests fail on arm64 because they call qemu-system-arm64

with cpu=max the qemu fails to boot the VM with recent EDK2.

The boot failure is caused by recent EDK2 enabling of LPA2 support and qemu is called with -cpu=max model that advertises LPA2. While this issue has to be investigated on EDK2/QEMU side, there is no reason for initramfs-tools stick to cpu=max that might be unstable for these tests.

This commit switch the cpu from max to the common model neoverse-n1 that is more stable.