qcow2: Make image inaccessible after failed qcow2_invalidate_cache()

If qcow2_invalidate_cache() fails, we are in a state where qcow2_close()
has already been completed, but the image hasn't been reopened yet.
Calling into any qcow2 function for an image in this state will cause
crashes.

The real solution would be to get rid of the close/open pair and instead
do an atomic reset of the involved data structures, but this isn't
trivial, so let's just make the image inaccessible for now.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
