cmdlib: Adapt the rpc calls

The following (blockdev) RPC calls are not converted yet (as they are
not straight forward or need more research):

* bdev_sizes
* blockdev_remove
* blockdev_shutdown
* blockdev_removechildren
* blockdev_close
* blockdev_getsize
* drbd_disconnect_net
* blockdev_rename (has already a special encoder, needs further research
  if needed at all)
* blockdev_getmirrorstatus (not sure if we have everywhere a clear link
  to the instance the disk belongs)
* blockdev_getmirrorstatus_multi (same here, further research)

Then special cases where we take care later in the patch series:

* blockdev_create (special cased)
* blockdev_find (special cased, like blockdev_create)

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>