Make qemu_template.sh work on MacOS
Line 14; The nproc command is only available on systems with GNU coreutils installed. The getconf _NPROCESSORS_ONLN alternative will work on a wider range of UNIX systems.
Line 114; The mktemp syntax used only works on GNU implementation.
Line 159; added hvf (MacOS) and tcg (no acceleration) options as a fallback. By doing this qemu-system-x86_64 will try to use kvm, but when it fails try hvf, and when that fails switch to the tcg accelerator.