systemd: Update path in unit file

When loading the unit file, systemd 240 logs:

systemd[1]: /usr/lib/systemd/system/spice-vdagentd.service:9: PIDFile=
references path below legacy directory /var/run/, updating
/var/run/spice-vdagentd/spice-vdagentd.pid →
/run/spice-vdagentd/spice-vdagentd.pid; please update the unit file
accordingly.

So update the path.

Signed-off-by: Christian Hesse <mail@eworm.de>
Acked-by: Frediano Ziglio <fziglio@redhat.com>