Problem: if you already downloaded QEMU, it will no longer untar it in case of a second install (because of the second if). Tip: the test is unnecessary, since wget -c already takes care of that (it takes 1-2 seconds to figure out the file was fully downloaded). For those with poor internet connections, it may even be helpful to leave it that way, as wget can resume the download in case the first attempt doesn't fully succeed.
Suggestion: 'M' to move the old installation directory (maybe some auto-naming, counter- or date-based, as you wish). Sometimes one may want to keep the changes.