running "systemctl start libvirtd.socket" I noticed a failure like:

Jun 01 11:25:48 byrd systemd[1]: [/usr/lib/systemd/system/libvirtd.socket:10]
Unknown lvalue 'SocketUser' in section 'Socket'
Jun 01 11:25:48 byrd systemd[1]: [/usr/lib/systemd/system/libvirtd.socket:11]
Unknown lvalue 'SocketGroup' in section 'Socket'
Jun 01 11:36:25 byrd systemd[1]: [/usr/lib/systemd/system/libvirtd.socket:11]
Unknown lvalue 'SocketGroup' in section 'Socket'

content is:
$ cat /usr/lib/systemd/system/libvirtd.socket
[Socket]
ListenStream=/var/run/libvirt/libvirt-sock
ListenStream=/var/run/libvirt/libvirt-sock-ro

; The following settings must match libvirtd.conf file in order to
; work as expected because libvirtd can't change them later.
; SocketMode=0777 is safe only if authentication on the socket is set
; up.  For further information, please see the libvirtd.conf file.
SocketMode=0777
SocketUser=root
SocketGroup=root