You can use VMs to configure as MONs for Ceph. But it is rather risky. If the storage the VMs are on becomes inaccessible for some reason your MONs will never come online thus potentially risking massive data loss. You can have majority of MONs in physical machine and some as VMs. We use all Proxmox nodes as MONs since the nodes are going to be active any way and they are not depended on any shared storage but their own OS drive. If you still want to use VM as MONs i think best way is to install an OS such as Ubuntu on VM and configure that as MON.