Fix incompatibility with libvncserver websockets handling

The previous commit:

  commit 7f4f2fe8
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Thu Jan 26 09:31:40 2017 +0000

    Add workaround to avoid hangs when connecting to SPICE

changed the code so that it would send the bytes "RFB " to the
server before we received its own greeting. This works fine for
VNC servers which follow the RFB protocol spec exclusively. The
libvncserver code though tries to implement websockets tunnelling
support on the same port as the normal RFB service. The way it
does this is by waiting 100ms after the client connects to see
if the client sends any data. If the client sends data, then it
tries to interpret this as an HTTP GET request to initiate the
websockets connection. This breaks when it sees our "RFB " bytes
being sent. Ideally the libvncserver would have just run a normal
RFB connection in this case, but that's not what happens, and
given the libvncserver code is in ...