usertools: skip empty categories in devices status

usertools: skip empty categories in devices status

If there aren't any devices of a particular category on user's
system, we still display them, which is bad for usability. Fix
devbind to not print out a category unless there are devices in
it.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Reviewed-by: David Hunt <david.hunt@intel.com>