CP-4500: Added handler skeleton for /export_raw_vdi

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CP-4500: Move helper functions to shared import/export code

Moved some functions in import_raw_vdi.ml into shared module importexport.ml
ready for implementing export_raw_vdi:

    * return_302_redirect
    * vdi_of_req

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CP-4500: Implemented export_raw_vdi.

This can be called via HTTP at <host>/export_raw_vdi?vdi=<vdi-uuid> using, for
example, curl and is the counterpart to import_raw_vdi.

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