core: OvfReader cleanup

- Rename GigabyteToBytes method to convertGigabyteToBytes
- Rename the methods in IOvfBuilder so that they will start with lower
  case letter instead of upper case letter
- Remove unnecessary parentheses

Change-Id: I30cb764dd0a303138b703f1cce1e943fb1e59fc8
Signed-off-by: Arik Hadas <ahadas@redhat.com>