Whitespace: Fix formatting in Xapi_vbd module.

The following produces the same before and after the commit.

    $ camlp4o -printer o -no_comments ocaml/xapi/xapi_vbd.ml | md5sum
    4d75d489ebdf8bb7b186baaba12a35bb  -

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CA-133780: Set VBD backend-kind on VBD creation

Getting this informaton from the VDI record in the Xenopsd metadata translation
layer caused CD-eject to hang because ejection is done by setting the VDI ref
for the VBD to NULL.

Instead, the SM can still set the backend-kind sm-config key which will be
placed in other-config of the VBD at creation. It can still be overwritten by
the user.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>