Hybrid boot, i.e. MBR+UEFI, is only supported on x86_64, but not
on aarch64, which only supports UEFI. Therefore, disable legacy,
i.e. MBR, boot for aarch64 and remove the corresponding BIOS boot
partition.
Since hybrid boot is always used on x86_64, the "uefi" parameter
is removed completely, together with the "legacy only" option on
x86_64. Thus we are left with a simple one to one mapping of
architecture to qemu assembler stage options in `qemuAssembler`.