If we are configured to use the systemd init script, also add support
for systemd socket activation. systemd will listen on the socket that is
used to communicate between the session agent and the system daemon.
When the session agent connects, the system daemon will automatically be
started. The socket will be enabled only if the required virtio-port
device exists. The socket is disabled when the device is removed.

This has a couple minor advantages to the previous approach:
  - For VMS that are not running a graphical desktop (and thus no
    session agents are running), the system vdagent daemon won't get
    started at all even if the spice virtio port is configured. Only the
    socket will be enabled. In the previous approach, the system daemon
    was started when the virtio device was added regardless of whether
    it was needed or not.
  - Solves issues related to switching between systemd targets. With the
    previous approach, when a user switches to a different...