For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:Vertigo.Red:branches:Virtualization/openSUSE_Tumbleweed/home:Vertigo.Red:branches:Virtualization.repo
zypper refresh
zypper install virt-manager
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:Vertigo.Red:branches:Virtualization/15.6/home:Vertigo.Red:branches:Virtualization.repo
zypper refresh
zypper install virt-manager
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:Vertigo.Red:branches:Virtualization/15.5/home:Vertigo.Red:branches:Virtualization.repo
zypper refresh
zypper install virt-manager