Comment # 7 on bug 914661 from Martin Pluskal

I have just tried this, and indeed unit file fails with symlink:
lojza:~ # systemctl start mgetty@ttyS0
lojza:~ # systemctl status mgetty@ttyS0
mgetty@ttyS0.service - mgetty on /dev/ttyS0
   Loaded: loaded (/usr/lib/systemd/system/mgetty@.service; disabled)
   Active: active (running) since Mon 2015-01-26 15:51:56 CET; 5s ago
     Docs: man:mgetty(8)
 Main PID: 10069 (mgetty)
   CGroup: /system.slice/system-mgetty.slice/mgetty@ttyS0.service
           ������10069 /usr/sbin/mgetty /dev/ttyS0

lojza:~ # systemctl stop mgetty@ttyS0
lojza:~ # ln -s /dev/ttyS0 /dev/modem
lojza:~ # systemctl start mgetty@modem
lojza:~ # systemctl status mgetty@modem
mgetty@modem.service - mgetty on /dev/modem
   Loaded: loaded (/usr/lib/systemd/system/mgetty@.service; disabled)
   Active: inactive (dead)
     Docs: man:mgetty(8)