qemu: fix bin/qemu-kvm on aarch64 + minor fixes

 * $out/bin/qemu-kvm should point to qemu-system-aarch64 on aarch64, libvirt expect it
 * makeWrapper codes are separated as some architectures might require additional command flags (https://github.com/NixOS/nixpkgs/issues/31606#issuecomment-349675127)
 * x86_64-on-i686 is not a native emulation and not supported by KVM, so it is removed from the list