When shutting down my system, there's a delay caused by libvirt-guests.sh.
The journal shows:
...
Aug 27 00:08:18 nuc8 systemd[1]: Stopped Permit User Sessions.
Aug 27 00:08:18 nuc8 systemd[1]: Stopped target Remote File Systems.
Aug 27 00:08:18 nuc8 systemd[1]: Stopped target User and Group Name Lookups.
Aug 27 00:08:18 nuc8 libvirt-guests.sh[7884]: Unable to connect to libvirt
currently. Retrying .. 10
Aug 27 00:08:19 nuc8 libvirt-guests.sh[7884]: Unable to connect to libvirt
currently. Retrying .. 9
Aug 27 00:08:20 nuc8 libvirt-guests.sh[7884]: Unable to connect to libvirt
currently. Retrying .. 8
Aug 27 00:08:21 nuc8 libvirt-guests.sh[7884]: Unable to connect to libvirt
currently. Retrying .. 7
Aug 27 00:08:22 nuc8 libvirt-guests.sh[7884]: Unable to connect to libvirt
currently. Retrying .. 6
Aug 27 00:08:23 nuc8 libvirt-guests.sh[7884]: Unable to connect to libvirt
currently. Retrying .. 5
Aug 27 00:08:24 nuc8 libvirt-guests.sh[7884]: Unable to connect to libvirt
currently. Retrying .. 4
Aug 27 00:08:25 nuc8 libvirt-guests.sh[7884]: Unable to connect to libvirt
currently. Retrying .. 3
Aug 27 00:08:26 nuc8 libvirt-guests.sh[7884]: Unable to connect to libvirt
currently. Retrying .. 2
Aug 27 00:08:27 nuc8 libvirt-guests.sh[7884]: Unable to connect to libvirt
currently. Retrying .. 1Can't connect to default. Skipping.
Aug 27 00:08:27 nuc8 systemd[1]: libvirt-guests.service: Succeeded.
Aug 27 00:08:27 nuc8 systemd[1]: Stopped Suspend/Resume Running libvirt Guests.
Aug 27 00:08:27 nuc8 systemd[1]: Stopped target Libvirt guests shutdown.
Aug 27 00:08:27 nuc8 systemd[1]: Stopped target System Time Synchronized.
Aug 27 00:08:27 nuc8 systemd[1]: Stopped target System Time Set.
Aug 27 00:08:27 nuc8 systemd[1]: Stopped target Network.
...
This behaviour started after upgrading libvirt-daemon-system from 5.2.0-2 to
5.6.0-1