Please configure the libvirtd package to install (or at least prompt to install) a polkit entry for KVM. This would be accomplished by creating the file:

/etc/polkit-1/localauthority/50-local.d/50-libvirt-ssh-remote-access-policy.pkla

That file should contain the following content:

[Remote libvirt SSH access] Identity=unix-group:libvirt Action=org.libvirt.unix.manage ResultAny=yes ResultInactive=yes ResultActive=yes

Please note: use of whitespace in this file can be temperamental.

(from redmine: issue id 10082, created on 2019-03-11)