It is not user documentation and should not be treated as such.
Documentation is available here.
The communication between oVirt guest agent and the oVirt engine
The engine directs requests for the guest agent to VDSM.
VDSM communicates with the guest using a VirtIO channel using plain text commands in JSON format. Each command is terminated by a new line character.
VDSM/Guest Agent message structure
Each message sent by VDSM has a mandatory field
__name__ which contains the command name.
All additional fields in the message are considered arguments to the command.