Commit 283281f broke compression by appending the argument last to the
tar command line. It needs to appear before the file.
Fix that and add a test.
[teg: add minor fix]
Similar to the existing test, but uses qemu-nbd to mount the generated
image.
Using unittest.TestCase.subTest() for now, which means that the tests
aren't very independent. I think this is fine in this case, because
we're testing images independently from each other, reusing the base
tree in the store.
