iotests: Test reverse sub-cluster qcow2 writes

iotests: Test reverse sub-cluster qcow2 writes

This exercises the regression introduced in commit
50ba5b2d994853b38fed10e0841b119da0f8b8e5.  On my machine, it has close
to a 50 % false-negative rate, but that should still be sufficient to
test the fix.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Tested-by: Stefano Garzarella <sgarzare@redhat.com>
Tested-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit ae6ef0190981a21f2d4bc8dcee7253688f14fae7)
 Conflicts:
	tests/qemu-iotests/group
*fix context deps on tests not in 4.1.0
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>