frontend: Fix SPICE-ActiveX upgrade issues

This patch basically reverts previous IE 11 workaround:

Syntax error (missing '+' to concatenate strings) in SpicePluginImpl
caused silent fail of activex plugin in IE. This patch addresses this.

This patch also removes IE11 workaround from SpicePluginImpl
(i.e. dummy <PARAM NAME="SecurePort" VALUE="0">). Current SPICE ActiveX
plugin already works on IE 11 without this workaround (moreover - it
crashes with it!).

Change-Id: I884eafed9985991f5f810378792fb45e00837a5e
Bug-Url: https://bugzilla.redhat.com/1081533
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>