The nbd device might not be ready after `qemu-nbd --connect` returns,
leading to access errors such as this further down:
sfdisk: cannot open /dev/nbd12: Inappropriate ioctl for device
Fix this by polling the device with `nbd-client --check <device>`.
Also, the nbd device might not be released after `qemu-nbd --disconnect`
returns. Fix this by using `nbd-client --disconnect`, which waits.
This introduces a new test dependency on nbd-client (in the ndb package
on Fedora).
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.
