qemu-iotests: Test 0-length image for mirror All behavior and invariant should hold for images with 0 length, so add a class to repeat all the tests in TestSingleDrive. Hide two unapplicable test methods that would fail with 0 image length because it's also used as cluster size. Signed-off-by: Fam Zheng <famz@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>