A few folks have been experiencing errors and long delays in adding a single-node (i.e. a 1 node cluster) cluster under management in Virtual Machine Manager.
This issue stems from the fact that VMM has an overcommitted property for each cluster. By default, a single-node cluster will always be overcommitted since any failure in that single node will cause the entire cluster and its Virtual Machines to go down. Such a failure does not go well with the concept of high availability.
A workaround for this issue is to set the node reserve failure to 0 in the cluster properties. (this can also be done using Windows PowerShell and the following cmdlet - "Set-VMHostCluster -VMHostCluster $VMHostCluster -ClusterReserve 0"). Once the cluster node reserve is set to 0, the cluster should now be in a healthy condition.