nbd: Detect servers that send unexpected error values

Add some debugging to flag servers that are not compliant to
the NBD protocol.  This would have flagged the server bug
fixed in commit c0301fc.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alex Bligh <alex@alex.org.uk>

Message-Id: <1463006384-7734-11-git-send-email-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

nbd/client.c

@@ -33,8 +33,10 @@ static int nbd_errno_to_system_errno(int err)
         return ENOMEM;
     case NBD_ENOSPC:
         return ENOSPC;
-    case NBD_EINVAL:
     default:
+        TRACE("Squashing unexpected error %d to EINVAL", err);
+        /* fallthrough */
+    case NBD_EINVAL:
         return EINVAL;
     }
 }