The error message after installation (of virt-manager) is the following: 

"Unable to connect to libvirt qemu://system. Verify that 'libvirtd' daemon is
running."

The steps to work around this problem is first install libvirt. Then start the
daemon with systemctl. [Side note: in the current release of Leap, adding the
root user to the group libvirt is no longer neccessary as it was until Leap
15.2 . It is still neccessary if I want to start virt-manager as non-root.]

These steps should be part of the installation. Please fix the package
dependencies and postinst scripts.