common/options,librbd/Utils: refactor RBD feature validation

common/options,librbd/Utils: refactor RBD feature validation

Move conversion of RBD features to/from string/uint64_t to helpers, and
use those from the option validator.

Keep the resulting features in integer form to avoid breaking Cinder.
Meh.

Change behavior from prior version:

- fail to parse if any of the specified features is invalid (previously
  we would parse the ones we understood and ignore the others).

Signed-off-by: Sage Weil <sage@redhat.com>