fs: implement VDI.snapshot in the example backend
fs: implement VDI.snapshot in the example backend

djs55/message-switch,djs55/message-switch-1,djs55/message-switch-1,djs55/message-switch,djs55/message-switch,xapi-project/message-switch,djs55/message-switch-1,xapi-project/message-switch,xapi-project/message-switch