frontend: Fix Spice ActiveX for IE 11

This patch provides quick fix of SPICE plugin invocation for Internet
Explorer 11.

Since IE 11 doesn't report itself as MSIE anymore and doesn't support
old IE DOM API, couple of small adjustments were needed.

Change-Id: I14760ddf688a29eb1c5ef896c20e194fdc7ed8ca
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1052873