ui: mix misleading comments & return types of VNC I/O helper methods

ui: mix misleading comments & return types of VNC I/O helper methods

While the QIOChannel APIs for reading/writing data return ssize_t, with negative
value indicating an error, the VNC code passes this return value through the
vnc_client_io_error() method. This detects the error condition, disconnects the
client and returns 0 to indicate error. Thus all the VNC helper methods should
return size_t (unsigned), and misleading comments which refer to the possibility
of negative return values need fixing.

Signed-off-by: Daniel P. Berrange <[email protected]>
Reviewed-by: Darren Kenny <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Message-id: 20171218191228[email protected]
Signed-off-by: Gerd Hoffmann <[email protected]>