iotests: fix remainining tests to work with LUKS

The tests 033, 140, 145 and 157 were all broken
when run with LUKS, since they did not correctly use
the required image opts args syntax to specify the
decryption secret. Further, the 120 test simply does
not make sense to run with luks, as the scenario
exercised is not relevant.

The test 181 was broken when run with LUKS because
it didn't take account of fact that $TEST_IMG was
already in image opts syntax. The launch_qemu
helper also didn't register the secret object
providing the LUKS password.

Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170626123510.20134-3-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
