vdagentd: Fix session lookup for new GNOME versions

New GNOME versions have started to manage the session using the systemd user instance. The effect of this is that the spice-vdagent running in the user session is forked off (indirectly) from the systemd user instance and does technically not belong to any session.

The correct way of handling this situation is to simply assume that the process belongs to the users graphical session. Add a heuristic to find the graphical session based on the UID, fixing spice-vdagent running on GNOME 3.34 with systemd integration.