libvirt users - Nov 2018 - guest on host's shutdown/reboot (problem?) - centos 7.5

hi guys

I have just two guest after which libvirt looks and when I shutdown 
those guests:

$ virsh shutdown $_dom

that domain takes little time to power down, as expected one would 
say(~20sec) But! I reboot the hosts and I have in my:

/etc/sysconfig/libvirt-guests :

ON_SHUTDOWN=shutdown

PARALLEL_SHUTDOWN=1

SHUTDOWN_TIMEOUT=300

I see libvirtd service shut down takes minutes, a few good minutes to 
deal with guests shutdown(I suppose?)

How could I dig into it and troubleshoot?

Would you have any idea as why libvirtd shutdown takes that long?

many thank, L.