As a virt-who user, I would like to add and remove hypervisors explicitly.
When a hypervisor is deleted on the server, either hosted or SAM/Sat6, candlepin saves a "deletion record" so that the hypervisor will never be shown again. This is useful to avoid having unmanaged hypervisors listed. However, if a hypervisor is accidentally deleted this deletion record itself must be deleted first before virt-who can add it back.
There is a katello hammer command ( http://projects.theforeman.org/issues/8028 ) to remove this record, however a better experience would be to enable this from virt-who itself.