qemu-iotests: update unsupported image formats in 194

Test 194 checks for 'luks' to exclude as an unsupported format,
However, most formats are unsupported, due to migration blockers.

Rather than specifying a blacklist of unsupported formats, whitelist
supported formats (specifically, qcow2, qed, raw, dmg).

Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Message-id: 23ca18c7f843c86a28b1529ca9ac6db4b35ca0e4.1510059970.git.jcody@redhat.com
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Max Reitz <mreitz@redhat.com>
