Install spice-vdagent from Jessie
Originally created by @intrigeri on #7671 (Redmine)
Due to https://bugs.debian.org/752953 having caused spice-vdagent to be removed from Jessie, we install spice-vdagent from sid in feature/jessie. Once it’s back in Jessie, we’ll want to revert this change (commit 66a3152a).