iotests: make 083 specific to raw

iotests: make 083 specific to raw

While testing the Python 3 changes which touch the 083 test, I noticed
that it would fail with qcow2.  Expanding the testing, I noticed it
had nothing to do with the Python 3 changes, and in fact, it would not
pass on anything but raw:

 raw: pass
 bochs: not generic
 cloop: not generic
 parallels: fail
 qcow: fail
 qcow2: fail
 qed: fail
 vdi: fail
 vhdx: fail
 vmdk: fail
 vpc: fail
 luks: fail

The errors are a mixture I/O and "image not in xxx format", such as:

  === Check disconnect before data ===

  Unexpected end-of-file before all bytes were read
 -read failed: Input/output error
 +can't open device nbd+tcp://127.0.0.1:PORT/foo: Could not open 'nbd://127.0.0.1:PORT/foo': Input/output error

  === Check disconnect after data ===

 -read 512/512 bytes at offset 0
 -512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 +can't open device nbd+tcp://127.0.0.1:PORT/foo: Image not in qcow format

I'm not aware if there's a quick fix, so, for the time being, it looks
like the honest approach is to make the test known to work on raw
only.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>