qemu-iotests: Test qcow2 image creation options

Just create lots of images and try out each of the creation options that
qcow2 provides (except backing_file/fmt for now)

I'm not totally happy with the behaviour of qemu-img in each of the
cases, but let's be explicit and update the test when we do change
things later.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>