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>