Writing zeroes to a qcow2 v2 images without a backing file results in an
unallocated cluster as of 61b30439.  197 has a test for COR-ing a
cluster on an image without a backing file, which means that the data
will be zero, so now on a v2 image that cluster will just stay
unallocated, and so the test fails.  Just force compat=1.1 for that
particular case to enforce the cluster to get allocated.

Fixes: 61b30439Signed-off-by: NMax Reitz <mreitz@redhat.com>
Message-Id: <20200727135237.1096841-1-mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>