When creating a qcow2 image, we didn't necessarily call
qcow2_update_header(), but could end up with the basic header that
qcow2_create2() created manually. One thing that this basic header
lacks is the feature table. Let's make sure that it's always present.

This requires a few updates to test cases as well.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>