Comment 11 for bug 1574566

Yes, I did. If you don't, you get a warning.

However, after thorough research, I noticed some leftovers from a previously installed libvirt 1.3.3 from sources over 1.3.1.
Uninstalling the latter did not uninstall everything, hence the mess.
After cleaning everything and reinstalling libvirt 1.3.1 without 1.3.3, I was even able to correctly start libvirt-bin without having to modify the service file.
I then modified it with NotifyAccess=all, systemctl daemon-reload & systemctl restart libvirt-bin: everything works fine so far:
systemctl status libvirt-bin
● libvirt-bin.service - Virtualization daemon
   Loaded: loaded (/lib/systemd/system/libvirt-bin.service; enabled; vendor preset: enabled)
   Active: active (running) since Tue 2016-04-26 10:03:09 CEST; 6s ago
     Docs: man:libvirtd(8)
           http://libvirt.org
 Main PID: 17108 (libvirtd)
    Tasks: 18
   Memory: 33.5M
      CPU: 666ms
   CGroup: /system.slice/libvirt-bin.service
           ├─14707 /usr/sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/lib/libvirt/libvirt_leaseshel
           ├─14708 /usr/sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/default.conf --leasefile-ro --dhcp-script=/usr/lib/libvirt/libvirt_leaseshel
           └─17108 /usr/sbin/libvirtd

Apr 26 10:03:09 samsung-ubuntu systemd[1]: Starting Virtualization daemon...
Apr 26 10:03:09 samsung-ubuntu systemd[1]: Started Virtualization daemon.
Apr 26 10:03:09 samsung-ubuntu dnsmasq[14707]: read /etc/hosts - 29 addresses
Apr 26 10:03:09 samsung-ubuntu dnsmasq[14707]: read /var/lib/libvirt/dnsmasq/default.addnhosts - 0 addresses
Apr 26 10:03:09 samsung-ubuntu dnsmasq-dhcp[14707]: read /var/lib/libvirt/dnsmasq/default.hostsfile