First of all, take a look at the general XO REST API documentation.
Then, take a look at xo-cli --help which explain how to use it to interact with the REST API.
You can list all VDIs at /rest/v0/vdis, snapshots are available at /rest/v0/vdi-snapshots.
You also have the ability to list the VDIs attached to a specific VM at /rest/v0/vms/:id/vdis.