test: test qemu assembler

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.

assemblers/org.osbuild.qemu

@@ -101,7 +101,7 @@ def main(tree, output_dir, options, loop_client):
     extra_args = []
 
     # raw and vdi don't suppport compression
-    if fmt != "raw" and fmt != "vdi":
+    if fmt not in ("raw", "vdi"):
         extra_args.append("-c")
 
     subprocess.run(["qemu-img", "convert", "-O", fmt, *extra_args, image, f"{output_dir}/{filename}"], check=True)