tests/qemu-iotests/184: Fix skip message for qemu-img without throttle

If qemu-img does not support throttling, test 184 currently skips
with the message:

  not suitable for this image format: raw

But that's wrong, it's not about the image format, it's about the
throttling not being available in qemu-img. Thus fix this by using
_notrun with a proper message instead.

Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20251014104142.1281028-2-thuth@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/184

@@ -51,7 +51,7 @@ run_qemu()
 }
 
 test_throttle=$($QEMU_IMG --help|grep throttle)
-[ "$test_throttle" = "" ] && _supported_fmt throttle
+[ "$test_throttle" = "" ] && _notrun "qemu-img does not support throttle"
 
 echo
 echo "== checking interface =="