qemu-guest-agent not starting
Description of problem:
Trace found in syslog :
```
syslog:Dec 11 13:45:08 mail systemd[1]: dev-virtio\x2dports-org.qemu.guest_agent.0.device: Job dev-virtio\x2dports-org.qemu.guest_agent.0.device/start timed out.
syslog:Dec 11 13:45:08 mail systemd[1]: Timed out waiting for device /dev/virtio-ports/org.qemu.guest_agent.0.
syslog:Dec 11 13:45:08 mail systemd[1]: qemu-guest-agent.service: Job qemu-guest-agent.service/start failed with result 'dependency'.
syslog:Dec 11 13:45:08 mail systemd[1]: dev-virtio\x2dports-org.qemu.guest_agent.0.device: Job dev-virtio\x2dports-org.qemu.guest_agent.0.device/start failed with result 'timeout'.
```
Steps to reproduce:
systemctl start qemu-guest-agent
Additional information:
Messages when installing the systemd unit : 
```
systemctl enable qemu-guest-agent
Synchronizing state of qemu-guest-agent.service with SysV service script with /lib/systemd/systemd-sysv-install.
Executing: /lib/systemd/systemd-sysv-install enable qemu-guest-agent
The unit files have no installation config (WantedBy=, RequiredBy=, Also=,
Alias= settings in the [Install] section, and DefaultInstance= for template
units). This means they are not meant to be enabled using systemctl.

Possible reasons for having this kind of units are:
• A unit may be statically enabled by being symlinked from another unit's
  .wants/ or .requires/ directory.
• A unit's purpose may be to act as a helper for some other unit which has
  a requirement dependency on it.
• A unit may be started when needed via activation (socket, path, timer,
  D-Bus, udev, scripted systemctl call, ...).
• In case of template units, the unit is meant to be enabled with some
  instance name specified.
 ```