There are two types of snapshots available in QEMU internal and external.   Internal - Requires QCOW2 images,  Doesn't support OVMF, but have seen a work around to change to rom, snap but you have to do a manual revert.   External - Supports only diskonly, no revert or delete options at present, manual revert needs VM to be stopped.   Which VM types are people looking to snapshot? Does anyone know if proxmox supports OVMF? I have asked the libvirt team if any ideas