Comment # 2 on bug 1197981 from Charles Arnold

I would recommend that running "Install Hypervisor and Tools" from the
menu or from the commandline "yast2 virtualization". Make sure that the
KVM Server and KVM Tools are selected (if desensitized, they are already 
installed). This will ensure that you have all the necessary packages for
virt-manager.

In the meantime, Leap 15.3 is out of support.