qemu-iotests: update unsupported image formats in 194

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 <[email protected]>
Signed-off-by: Jeff Cody <[email protected]>
Message-id: 23ca18c7f843c86a28b1529ca9ac6db4b35ca0e4.1510059970[email protected]
Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Denis V. Lunev <[email protected]>
Signed-off-by: Max Reitz <[email protected]>