iotests: blacklist 194 with the luks driver

The 194 test has a lot of code that assumes a simple image file. Rewriting
this to work with luks is possible, but non-trivial, so blacklist the
luks format for now.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20170901105434.3288-3-berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Fam Zheng <famz@redhat.com>
[eblake: commit message typo fixed]
Reviewed-by: Kashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>