jsonrpc: VM_snapshot

Align naming between API.py and schema.

Change-Id: Id8af5bf7d0b15e98ad1352bffe3ee41ef2bccbe2
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/35555
Reviewed-by: Oved Ourfali <oourfali@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Adam Litke <alitke@redhat.com>