xenbus: xenbus_gather()/xenbus_scanf() usage adjustments

xenbus: xenbus_gather()/xenbus_scanf() usage adjustments

- don't use xenbus_gather() for just a single, non-string item (as it
  doesn't do format checking on its inputs)
- grant references don't need to be parsed as "long", since
  grant_ref_t
  is a typedef of uint32_t
- in the frontend protocol determination logic in blkback and blktap,
  don't parse into a local buffer - use the allocated string directly

Signed-off-by: Jan Beulich <jbeulich@novell.com>