checker: ignore fields marked unused

While comparing qemu-1.0 json output with qemu-2.1, a few fields got
marked unused.  These need to be skipped over, and not flagged as
mismatches.

For handling unused fields, the exact number of bytes need to be skipped
over as the size of the unused field.

Currently, only the term "unused" is matched.  When more field names
turn up, this will have to be updated based on the whitelist matching
method to match more such terms.

Signed-off-by: Amit Shah <amit.shah@redhat.com>
