CA-397268: vbd.create failed: The device name is invalid

In the commit 62db5cb, the device name validation was consolidated with
the function which converts name to device number. Particularly, the
function "Device_number.of_string" is used for both the validation and
the conversion.

But one issue was introduced in the validation is that the "None" value
returned from "Device_number.of_string" is considered as valid. This
causes the error "the device name is invalid".

This commit just fixes this issue.

Signed-off-by: Ming Lu <[email protected]>

ocaml/xapi/xapi_vbd_helpers.ml

@@ -377,7 +377,7 @@ let clear_current_operations ~__context ~self =
 (** Check if the device string has the right form *)
 let valid_device dev ~_type =
   dev = "autodetect"
-  || Option.is_none (Device_number.of_string dev ~hvm:false)
+  || Option.is_some (Device_number.of_string dev ~hvm:false)
   ||
   match _type with
   | `Floppy ->