CP-6172: Don't specify VBD backend-kind by default

Rather than defaulting this value (meaning we have to keep parity with
xenopsd), leave the default to be determined by xenopsd itself by not
specifying a backend-kind.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #1603 from simonjbeaumont/cp-6172-xapi-part

CP-6172: Don't specify VBD backend-kind by default