CA-119317: Move backend-kind to VDI.other_config

This way the SM driver can set this and special case where necessary for
certain SR types.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CA-119317: Consider VDI.sm_config:backend-kind

Allow a way of an SM driver to configure the VBD backend to be used for this
VDI. This is done by setting the backend-kind key in the sm_config field.

This value will be used but will still be overridden if a value appears in
other-config (user-configurable).

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