Remove VDI.{detach,activate,deactivate}

The most basic kind of SR only needs to be able to convert a
VDI into some kind of protocol URI which can be separately
'opened' etc

I think if a storage implementation needs 'activate' then it
needs to create its own 'initiator' and do a deeper integration.

Signed-off-by: David Scott <dave.scott@citrix.com>