qemu-iotests: Fix test 031 for qcow2 v3 support

qcow2.py must be updated to work with version 3 images at all, the
output has changed since the feature table extension has been added, and
version 2 and version 3 images can't possibly have the same test output.

Change the test case to completely ignore IMGOPTS and run the test for
both compat=1.1 and compat=0.10 regardless of the ./check command line.

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