frontend: hotfix - spice activex plugin upgrade in ie11

This hotfix handles broken logic for retrieving spice activex version on
in the engine which prevented ie11 clients to install updates.

Change-Id: I86615c88545ba26282c4f891389e4b94e85de117
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1072409