Add Datapath.open and Datapath.close
Add Datapath.open and Datapath.close

- Datapath.open will be called before the first Datapath.attach when
  a VM is to be started.
- Datapath.close will be called after the final Datapath.detach when
  a VM is to be shutdown.
- Datapath.open will take a 'persistent' argument which, if true, means
  the data should be preserved; otherwise the data will be thrown away
  (aka 'clone on boot')

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@eu.citrix.com>

xapi-project/message-switch,xapi-project/message-switch,xapi-project/message-switch