frontend: Add secure-channels to .vv file

This patch adds secure-channels line to SPICE .vv descriptor.

Change-Id: Ie3868ed99c915e0444c94f502075bbe1e8b05339
Bug-Url: https://bugzilla.redhat.com/1043398
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>