[root@ganeti01 ~]# cat agent.xml
<channel type='unix'>
   <target type='virtio' name='org.qemu.guest_agent.0'/>
</channel>

[root@ganeti01 ~]# virsh attach-device --live --config gestionweb agent.xml
error: Failed to attach device from agent.xml
error: internal error: no virtio-serial controllers are available

[root@ganeti01 ~]# rpm -q libvirt qemu
libvirt-1.2.17-13.el7_2.2.x86_64
qemu-2.0.0-1.el7.6.x86_64
[root@ganeti01 ~]# cat /etc/redhat-release
CentOS Linux release 7.1.1503 (Core)