rc.apparmor.functions: only use systemd-detect-virt if it's present
This is a follow-up on !812, which added a call to systemd-detect-virt.
Everywhere else we don't assume that program is present,
and first check if it's there before we run it.
Let's do the same here.
Merge Speed up list creations, and change lists to tuples where appropriate.
Tuples [are quite a bit faster than lists](https://stackoverflow.com/a/22140115). If you don't need mutability, a tuple is almost always the better collection to choose. This merge request changes lists to tuples where appropriate, and speeds up some list creations by changing `list()` calls into `[]` literals. It also changes a few function calls to use tuple unpacking, i.e. `func(*tup)`, as opposed to `func(tup[0], tup[1], ...)`. This yields yet another performance improvement, as well as a readability improvement.
This partially addresses issue #239. There are still some remaining instances where opened files are not properly closed, e.g. the `NamedTemporaryFile` in `utils/apparmor/config.py`.
