Makefile: Use variable for qemu

This way users can specify which backend to use without having to change
the Makefile itself.

README.md

@@ -16,6 +16,8 @@ No! PonyDOS uses its own 512-byte kernel, built from scratch.
 
 To build PonyDOS from scratch, on an environment with `nasm` and `python3`,
 run `make`. `make run` runs the floppy disk image `ponydos.img` with QEMU.
+By default, `qemu-system-i386` is invoked. If you want to use a different
+backend, use `make QEMU=<backend> run`.
 
 ### How do I contribute?